ALLERGY

The term “Allergy” was born on July 24, 1906 in the Münchener Medizinische Wochenschrift as “specifically altered reactivity of the organism”. Today, we define allergy as immunological hypersensitivity that can lead to a variety of different diseases via different pathomechanisms and thus different approaches in diagnosis, therapy and prevention can be taken. 

Allergy manifests in form of various different conditions such as anaphylaxis, urticaria, angioedema, allergic rhinoconjunctivitis, allergic asthma, serum sickness, allergic vasculitis, hypersensitivity pneumonitis, atopic dermatitis (eczema), contact dermatitis and granulomatous reactions, food or drug induced hypersensitivity reactions. Allergies can be seen in almost every organ, most commonly seen in the skin and the mucous membranes. 

Types of Hypersensitivity Reactions

Immediate hypersensitivity- the symptoms are manifest within minutes or hours after a sensitized recipient encounters antigen. In immediate hypersensitive reactions, different antibody isotypes induce different immune effector molecules. IgE antibodies, for example, induce mast-cell degranulation with release of histamine and other biologically active molecules. IgG and IgM antibodies, on the other hand, induce hypersensitive reactions by activating complement.

Delayed-type hypersensitivity (DTH) is so named in recognition of the delay of symptoms until days after exposure. In delayed-type hypersensitivity reactions, the effector molecules are various cytokines secreted by activated T_H or T_C cells.

ALLERGENS

The term allergen is used to describe any substance therein capable of stimulating production of immunoglobulin E (IgE) (a property known as allergenicity) in a genetically disposed individual.

Allergens often are found in whole organisms or structures such as pollen grains, seeds, fruits, fungal conidia (spores), and mite and insect fecal pellets in either intact or in mechanically damaged forms. They also are actively secreted in saliva or sweat from various domesticated animals or passively released from pollens and feces on contact with moisture, reflecting their function within the host. Clinically important allergen sources contain mixtures of (glyco)proteins; the most complex of these are pollens, fungi, seeds, and mites, with the least complex being animal danders and urine and those from occupational sources.

Patients usually produce IgE to more than one protein in any given source. A hierarchy of responsiveness has been observed, however, with some allergens being recognized by a greater percentage of exposed individuals than others. Allergens in a particular source that are recognized by more than 50% of allergic individuals are termed major, but some of those considered minor on a population basis may, of course, be clinically significant for a particular person. The capacity of susceptible individuals to produce allergen-specific IgE reflects the combined influences of genotype, allergen concentration and solubility, source complexity, and innate biochemical properties.

Potential allergens enter the body via routes such as them respiratory and gastrointestinal tracts, but they also may be injected (both naturally, as through envenomation and insect bites, and iatrogenically) or absorbed percutaneously. The route of exposure influences the types of allergic symptoms subsequently experienced, with exposure to aeroallergens giving rise to respiratory symptoms, in contrast with those ingested, injected, or absorbed, which cause localized gastrointestinal or dermal symptoms or more generalized systemic symptoms. In addition, individuals may be exposed to allergens resulting from gastrointestinal or respiratory infections due to fungal or helminthic pathogens. Finally, certain host proteins also may be allergenic and are termed autoallergens.

TYPES OF ALLERGENS

A.    AEROALLERGENS

Aeroallergens constitute the most common cause of allergic disease and are derived from pollens, fungal spores, insect and mite feces, animal danders, and dusts.

• GRASS POLLEN AEROALLERGENS

Pollen allergens arise from pollination processes occurring in wind-pollinated (anemophilous) angiosperms and gymnosperms, including trees, herbaceous dicotyledons (weeds), and grasses. Exposure reflects the types of plants growing in a particular location, as well as pollen-specific characteristics such as buoyant density, ease of dispersion, and profusion.

95% of allergy-relevant grass species belong to three subfamilies; Pooideae, Chloridoideae and Panicoideae

TAXONOMY OF GRASS POLLEN ALLERGENS

• HERBACEOUS DICOTYLEDON SPECIES–DERIVED POLLEN AEROALLERGENS

Pollens from herbaceous dicotyledon species also may be allergenic, and the allergens associated with members of the Asteraceae, Urticaceae, Chenopodiaceae, and Brassicaceae families.

The clinically important Asteraceae species are ragweed, mugwort, sunflower, and feverfew, whereas the important Urticaceae and Brassicaceae species include the wall pellitory and oilseed rape and turnip respectively.

•  TREE POLLEN–DERIVED AEROALLERGENS

Tree pollen allergens are derived from both angiosperms (flowering trees) and gymnosperms (nonflowering conifers).

The most clinically relevant sources of tree pollen allergens are found among Fagales, Oleaceae, and Cupressaceae plants, which are widely distributed worldwide.

Bet v 1-like proteins, Ole e 1- like proteins, and pectate lyases/ polygalacturonases represent the major pollen allergens of Fagales, Oleaceae, and Cupressaceae, respectively.

Bet v 1-like allergens are responsible for allergic cross-reactions among Fagales pollen and various fruits and vegetables, a clinical condition referred to as oral allergy syndrome.

•  FUNGI-DERIVED AEROALLERGENS

Fungi are significant sources of allergens and of the more than 180 fungal species shown to produce allergenic proteins, those of the Ascomycota and Basidiomycota phyla are clinically important.

All use airborne conidia (spore) dispersal for reproduction and are often produced in concentrations exceeding those seen with pollens. In addition to spores, allergens are found in mycelia, fragmented hyphae, and yeast forms, and such sources will be relevant in conditions such as allergic bronchopulmonary aspergillosis (ABPA) and immediate and delayed-type dermal infections involving Aspergillus fumigatus, Candida albicans, Trichophyton species, and Malassezia furfur. A majority of allergens are proteins or glycoproteins, but mannans from C. albicans and M. furfur also may be allergenic.

Fungi of clinical importance include Aspergillus, Penicillium, Cladosporium, and Alternaria species.

The major allergens from the three clinically important species—with regard to either respiratory disease or ABPA—include Alt a 1, 2, and 13, Asp f 1, 2, and 4, and Cla h 1, respectively. 

•  ANIMAL DANDER–DERIVED AEROALLERGENS

Clinically important animals in either domestic or occupational settings are cats, dogs, cows, rats, mice, horses, rabbits, gerbils, and guinea pigs and their allergens are derived from dander, epithelium, fur, urine, or saliva.

 

Allergy to pets is among the most common cause of asthma and polysensitisation to several pets a risk factor for severe problematic asthma. 

• ARTHROPOD-DERIVED AEROALLERGENS

The main arthropod allergen sources are from species of the Insecta and Arachnida classes and allergy may arise either in the home or in scientific institutions where many arthropods are reared or studied. Of the arthropods, house-dust mites and cockroaches are clinically important, and allergens are derived from whole bodies, salivary secretions, and fecal pellets accumulating in house dust or in dust generated by the rearing of insects. Many such allergens are gut-derived and are therefore present in fecal pellets, although other sources such as saliva, body debris, and secretions may contain allergens.

Insect-Derived Aeroallergens: In midges, the major allergens are hemoglobins (e.g., Chi t 1) whereas in cockroach, the major allergens include the group 1 nitrile-specifier proteins (e.g., Bla g 1), the group 2 inactive aspartic proteases (e.g., Bla g 2), the group 4 lipocalins (e.g., Bla g 4), the group 5 glutathione S-transferases (e.g., Bla g 5), the group 6 troponin C proteins (e.g., Bla g 6), the group 7 tropomyosins (e.g., Per a 7), hexamerin (e.g., Per a 3), an insect storage protein belonging to the hemocyanin superfamily (e.g., Per a 3), and the group 9 arginine kinases (e.g., Per a 9).  

 

Mite-Derived Aeroallergens: Mite species are ubiquitous, but the most clinically important species belong to the Pyroglyphidae, Acaridae, Glycyphagidae, and Echimyopodidae families.

 

• OCCUPATION-ASSOCIATED AEROALLERGENS 

Occupational allergens include fungal, bacterial, and mammalian hydrolytic enzymes; egg powder; latex products; flours derived from rice, wheat, barley, and rye seeds; castor bean and mustard seeds; green coffee beans; ispaghula; and soybeans. Allergy may occur in industries in which they are produced, used, or added to other products such as washing detergents, pancreatic supplements, and doughs from bulk storage. The occupation involved often is used to describe the resulting condition—for example, baker’s asthma with flour allergens. In addition, exposure may be more geographically widespread, as illustrated by the outbreaks of asthma in various port cities in the United States and Spain when soybeans were transferred to silos from cargo ships, resulting in allergy to soybean husk allergens (e.g., Gly m 1). Although the respiratory tract is the major route of exposure, sensitization and provocation also may occur percutaneously, as, for example, with natural latex rubber allergens present in latex gloves.

1.  Enzyme Aeroallergens Derived from Fungal, Bacterial, and Mammalian Sources

Hydrolytic enzymes used in a variety of industrial processes to facilitate the breakdown complex biologic polymers such as proteins and polysaccharides may be allergenic. The clinically important hydrolytic enzyme aeroallergens include the bacterial subtilisins and amylases used in the detergent industry, the mammalian serine proteases (e.g., trypsin, chymotrypsin, pepsin) used in pancreatic supplements in the treatment of cystic fibrosis patients, the fruit cysteine proteases papain (e.g., Car p 1) and bromelain (Ana c 2) used in the pharmaceutical industry (and which may be ingested allergens), and the fungal amylases (e.g., Asp o 21) and various other carbohydrases such as β-xylosidase (Asp f 14), cellulase, and glucoamylase used in the baking industry. The major egg aeroallergens in egg powder are derived from either the white or the yolk. The egg white allergens include ovomucoid (Gal d 1), ovalbumin (Gal d 2), conalbumin (Gal d 3), and lysozyme (Gal d 4), whereas that in yolk is α-livetin (chicken albumin, Gal d 5). The most frequently recognized egg allergens, however, are Gal d 3 and Gal d 5. 

2.     Seed-Derived Aeroallergens

The major seed-derived aeroallergens are proteins involved in defense, storage, or metabolism and allergy arises from exposure to the flours produced from them. The major defense-related allergens include 12- to 15-kDa amylase/ trypsin inhibitory albumin proteins (e.g., Poaceae group 15), α-amylase inhibitors (e.g., Poaceae group 28, 30) and the nsLTP allergens (e.g., Gly m 1), defensins (e.g., Gly m 2), and trypsin inhibitors (e.g., Gly m TI) from the husks of soybeans. The abundant hydrophobic and cysteine-rich nsLTPs from soybeans are seed surface proteins that reduce wettability and are therefore thought to play a role in defense. A number of minor allergens have been described, particularly from the Poaceae family, and include gliadins (e.g., Tri a 19, Tri a 21) and thioredoxin (e.g., Tri a 25).

3.   Natural Rubber Latex Aeroallergens

Allergy to proteins present in natural rubber latex from the lactiferous rubber tree (Hevea brasiliensis) arises through airborne, percutaneous, or parenteral routes. Airborne exposure results from the attachment of latex proteins to the dry lubricants, such as cornstarch, used to enable donning of gloves, in contrast with parenteral exposure caused by leaching from instruments used in surgical procedures. The major allergens from latex include rubber elongation factor (Hev b 1, 3), endo-1,3-β-glucosidase (Hev b 2), microhelix component (Hev b 4), a protein of unknown function (Hev b 5), prohevein (Hev b 6), esterase (Hev b 13), and chitinase (Hev b 14). The minor allergens include a patatin-like protein (Hev b 7), enolase (Hev b 9), manganese superoxide dismutase (Hev b 10), and profilin (Hev b 8). 

B.     FOOD / INGESTED ALLERGENS

A variety of sources contain allergens provoking IgE-mediated symptoms after ingestion in sensitized individuals, but seven appear to be most clinically important and account for more than 90% of food-induced allergy. These are, in decreasing order of frequency, eggs, peanut, milk, nuts, soy, fish, and wheat. Recently, however, allergy to animal meat proteins has been shown to be important.

Depending on the route of sensitization, immediate-type food hypersensitivities are either a result of reactivity to food allergens through the gastrointestinal tract (class I allergens) or the result of secondary sensitization to cross-reactive food allergens mainly due to primary sensitization to homologous pollen allergens via the respiratory tract (class II allergens). Class I allergens are often resistant to heat, degradation and digestion. Class II allergens are mainly labile and easily degradable. According to these characteristics the clinical manifestation is influenced by the type of allergens to which an individual is sensitized. The class I allergens have a higher potential to induce severe reactions compared to the easily degradable class II food allergens, which induce often symptoms restricted to the oral cavity.

Ingested food proteins may evoke a range of symptoms such as dermatitis, asthma, anaphylaxis, angioedema, and abdominal symptoms, and in this regard, certain sources are often associated with specific allergic manifestations. For example, peanuts, fish, and crustaceans are often associated with anaphylaxis whereas egg and milk are associated with atopic dermatitis, and wheat allergens may be associated with exercise-induced anaphylaxis. Food allergens, particularly the pathogenesis-related proteins, are resistant or stable to heat and acid, as well as being more resistant to proteases than nonfood allergens, thereby facilitating their entry through the gut mucosa.

• ANIMAL-DERIVED INGESTED ALLERGENS

Ingested mammalian meat allergens include the serum albumins (e.g., Bos d 6), immunoglobulins (e.g., Bos d 7), and transferrin (Bos d 8), whereas in cow milk, the allergens include α-lactalbumin (Bos d 4), β-lactoglobulin (Bos d 5), and casein (Bos d 8), with the last being particularly important. In addition, recent data highlight the role of red meat–associated Galα1-3Galβ1-4GlcNAc-R moieties in delayed anaphylaxis. In this unusual form of anaphylaxis, it is likely that the original sensitizing allergen is not red meat but rather exposure to the tick Amblyomma americanum.  

In fish, the major allergens are the calcium-binding parvalbumins (e.g., Gal c 1), whereas in shellfish and mollusks (shrimp, crab, snails), the major allergens are tropomyosins (e.g., Pen a 1) or actin-associated arginine kinases (e.g., Pen m 2). Similar allergens from different species show high sequence similarity and are therefore immunologically cross-reactive and play a role in oral allergy syndrome (OAS). Regarding tropomyosins, this cross-reactivity extends to the homologous allergens from invertebrates, such as cockroaches and dust mites, and the nematode parasite Anisakis simplex, for which the sequence similarity is approximately 80%.

• SEED-DERIVED INGESTED ALLERGENS 

Some of the major ingested seed-derived allergens are similar to those described previously as occupational aeroallergens. The clinically important are those derived from peanuts and include the major vicilin storage protein (Ara h 1), conglutin (Ara h 2), and peanut agglutinin (Ara a agglutinin). The minor peanut allergens include profilin (Ara h 5), the glycinin allergens (Ara h 4, 5) and proteins similar to conglutins (Ara h 6, 7). The most important ingested allergens from soybean include Gly m 1, Gly Bd 28K, and Gly m 3. Gly m 1 is an nsLTP, whereas Gly Bd 30K/P34 is a cysteine protease that shows homology with members of the papain family including the house-dust mite allergen Der p 1. By contrast, Gly m 3 is a profilin, and Gly m Bd 28K shows homology with a range of vicilin-related storage proteins, including Ara h 1, and may possess chitin-binding properties. Allergens belonging to the vicilin family are also present in walnut (Jug r 2) and coconuts.

 

• FRUIT- AND VEGETABLE-DERIVED INGESTED ALLERGENS 

This form of sensitivity is referred to as Oral Allergy Syndrome or pollen-food syndrome and is due to the cross-reactivity between proteins in the respiratory allergen source and those in food, usually pan-allergens. The condition is associated predominantly with uncooked food, because cooking and processing generally result in protein denaturation. Clinical manifestations range from mild oropharyngeal symptoms to severe, systemic reactions.

The allergens associated with latex-fruit allergies are endo-1,3 beta glucosidases, patatin-like proteins, profilins, and non-specific lipid transfer proteins, and those associated with arthropod crustaceans are tropomyosins; with egg-egg, lysozymes are involved. The degree of similarity between proteins required to elicit immunologic responses varies among different protein families and ranges from 35% to 60%.

C.    INJECTED ALLERGENS 

Injected allergens typically are arthropod-derived. Both venoms and saliva from biting and stinging insects are complex and serve important defensive and nutritional functions for the host such as inhibition of blood clotting (thrombin inhibition, inhibition of agonist-induced platelet aggregation), kinin liberation, neurotoxicity, immunomodulation, and tissue permeability.

The most important injected insect allergens are derived from the stinging or biting hymenopteran, dipteran, and hemipteran insects and include venoms from bees, wasps, hornets, paper wasps, and ants and salivary proteins from mosquitoes, flies, ticks, and fleas.

• VENOM-DERIVED ALLERGENS 

The major venom allergens from bees, wasps, and hornets include the group 1 A1 phospholipases (e.g., Dol m 1, Pol a 1), the group A2 phospholipases (e.g., Api m 1, Bom p 1), the group 2 hyaluronidases (e.g., Api m 2, Dol m 2), the group 3 acid phosphatases (e.g., Dol m 3, Api m 3) and proteases.

The phosphatases are likely to be involved in inhibition of platelet aggregation, whereas the hyaluronidases, which cleave extracellular matrix, result in increased tissue permeability, thereby facilitating venom spread. 

• SALIVA-DERIVED ALLERGENS 

Saliva from hematophagous insects such as fleas, mosquitos, and house flies contains a variety of minor and major allergens.

 

The major allergens include apyrases (e.g., Tab y 1), enzymes that catalyze the breakdown of ATP to release phosphate (an important platelet aggregation agonist), a Vespidae group 5 homolog (e.g., Cte f 2), hyaluronidase (e.g., Tab y 2), and lipocalins (e.g., Tria p 1), together with proteins of unknown function.

D.   PATHOGEN-DERIVED ALLERGENS AND AUTOALLERGENS 

A limited number of potentially pathogenic organisms are associated with allergic diseases. These include fungi, which may colonize patients with preexisting conditions including asthma and cystic fibrosis; bacteria associated with atopic dermatitis; and helminthic parasites ingested in contaminated raw seafood.

• HELMINTH-DERIVED ALLERGENS 

Major helminth allergens include the polyprotein lipid-binding allergen (Asc s 1), tropomyosin (Asc l 3) from Ascaris species, various protease inhibitors (Ani s 1 and 4), paramyosin (Ani s 2), and others of unknown function (Ani s 7) from Anisakis simplex (e.g., the serine proteases from Trichinella spiralis, cyclophilin from Echinococcus granulosus, and a range of protease inhibitors from Anisakis simplex (Ani s 1), E. granulosus (Ag B), and Schistosome species. 

 

•  FUNGI- AND BACTERIA-DERIVED ALLERGENS

The allergens associated with the fungal diseases usually are the same as those associated with allergic diseases. Regarding bacterial allergens, the enterotoxins and toxic shock syndrome toxin (TSST-1), as well as fibronectin-binding protein, flagellin, ribosomal proteins, and DNA binding proteins from Staphylococcus, are allergenic, but Staphylococcal enterotoxins A and B (SEA, SEB) and TSST-1 are most prominent. The S. aureus enterotoxins are potent T cell mitogens (termed superantigens) that cross-link MHC class II molecules and the variable region domain of the T cell receptor β chain with the potential to stimulate T cell proliferation in a non–antigen-specific manner. They stimulate IgE production in patients with asthma, rhinitis, and atopic dermatitis.

• HUMAN AUTOALLERGENS 

The study of human autoallergens arose from observations that human dander caused immediate wheal and flare reactions in allergic individuals. Subsequent molecular studies defined a number of autoallergens, revealing that they arose primarily in patients with atopic dermatitis or ABPA.

They are divided into two categories—namely, those that share significant homology with allergens from environmental sources such as pollen and fungi and are thus classified as crossreactive allergens (e.g., Hom s 4), and those that do not (e.g., Hom s 1), indicating that they may be genuine autoallergens. 

ROLE OF IgE ANTIBODIES IN ALLERGY

Discovery of IgE

The existence of a human serum factor that reacts with allergens was first demonstrated by K. Prausnitz and H. Kustner in 1921. The local wheal and flare response that occurs when an allergen is injected into a sensitized individual is called the P-K reaction. Because the serum components responsible for the P-K reaction displayed specificity for allergen, they were assumed to be antibodies, but the nature of these P-K antibodies, or reagins, was not demonstrated for many years.

Experiments conducted by K. and T. Ishizaka in the mid- 1960s showed that the biological activity of reaginic antibody in a P-K test could be neutralized by rabbit antiserum against whole atopic human sera but not by rabbit antiserum specific for the four human immunoglobulin classes known at that time (IgA, IgG, IgM, and IgD). In addition, when rabbits were immunized with sera from ragweed-sensitive individuals, the rabbit antiserum could inhibit (neutralize) a positive ragweed P-K test even after precipitation of the rabbit antibodies specific for the human IgG, IgA, IgM, and IgD isotypes. The Ishizakas called this new isotype IgE in reference to the E antigen of ragweed that they used to characterize it.

The discovery of IgE has had a significant impact on the diagnosis and management of allergic disease, enabling clinicians to differentiate between IgE-mediated allergic diseases and other hypersensitivity reactions, and to manage allergic diseases according to their underlying mechanisms. Tests became available that allowed a more simple and reliable diagnosis covering a very broad spectrum of allergens

How does allergy develop?

It is true in most cases that a person cannot have an allergic reaction to a substance that he or she has never come across. Encountering an allergen once is therefore usually necessary to develop an allergy. The process through which a person’s body becomes sensitive to a given allergen is known as sensitisation.

Sensitisation

When allergens enter the body, antigen presenting cells (immune cells that capture incoming substances and present them to other immune cells, initiating a cascade of immune responses) at body surfaces, capture and present them to immune cells, particularly T cells (in a similar manner as if the allergen was a foreign invading microbe). Through a number of immune interactions between T cells and B cells, B cells produce allergen-specific IgE antibodies. Once released into the blood, IgE binds to mast cells (the major allergy immune cell), as well as other immune cells such as basophils. Some, but not all, individuals who are sensitised will develop an allergic reaction on re-exposure to the allergen. 

Re-exposure to allergen

It is possible for an individual to go their whole life carrying allergen-specific IgE bound mast cells without ever experiencing an allergic reaction or even being aware of the allergy. However, upon re-exposure to the offensive allergen, binding of the allergen to IgE on mast cells can initiate an aggressive and immediate immune response. Mast cells are granular cells, meaning they contain many secretory granules which, when activated, release their contents into the blood stream.

In the case of allergy, binding of an allergen to IgE-mast cells results in their rapid degranulation and the release of inflammatory compounds, including histamine, which contribute to local inflammation and the symptoms associated with allergy. In the respiratory tract, for example, inflammation results in mucus secretion and vasodilation in the nose and lungs, which can lead to wheezing and difficulty breathing. In order to offset the immediate symptoms of an allergic reaction, people with allergies can take anti-histamines, a class of drug that limits the action of histamine on the body. In extreme cases of anaphylaxis, patients may require an adrenaline injection. In addition to early-phase symptoms, a number of symptoms can occur several hours after exposure to the allergen and can even last upwards of weeks. Late-phase reactions can result in similar symptoms, but also tissue destruction and continued immune cell recruitment. 

CYTOKINES IN ALLERGY

Cytokines are soluble proteins or peptides that act as the hormones - messengers - of the immune system and between other cells of the body. They confer cell-to-cell communication which may take place between adjacent cells (juxtacrine) or cells in different organs of the body (para- or endocrine). A cytokine signal is delivered via a receptor on the surface of a cell, and since different cells may express the same receptor, a cytokine can have several functions (pleiotropy) depending on the target cell. Also, a target cell may have receptors for several similar cytokines allowing for redundancy.

There are more than 100 described cytokines, some are named after where they were first found and/or their function, such as thymic stromal lymphopoietin (TSLP), others after the first function identified, like Granulocyte-Macrophage Colony Stimulating Factor (GM-CSF), and some may even have several names given to them by different research groups.

Various groups of cytokines are responsible for the different phases of the allergic sensitization (building up the allergic immune response) and elicitation (reactions upon exposure to an allergen):

The sensing cytokines: IL-33, IL-25, TSLP. These are released from the epithelial cells of the mucous membranes and signals to the allergen- presenting dendritic cells to take up incoming allergens and bring them to the lymph nodes.

The T-cell instructing cytokines will instruct undifferentiated T helper (CD4+) cell to develop into different kinds of cells, each of them equipped for different kinds of immune response: IL-12 and γ-interferon will produce T helper cells type 1 (Th1) that helps fighting bacteria and viruses, IL-4 leads to Th2 cells which fights large multicellular parasites like worms, but unfortunately also create the allergic immune response.

T-cell effector cytokines in allergy are the cytokines by which Th cells exerts their action: Th2 cells release IL-4 and IL-13 which instructs B-cells to produce the allergy antibody IgE, IL-5 that stimulates the bone marrow to form the eosinophilic granulocyte, and IL-9 that together with IL-13 creates the allergic inflammation e.g. in the lung as is the case in asthma.

The resolving cytokines such as IL-10 and Transforming Growth Factor (TGF-β) comprise a small but important group of cytokines that down-regulates the allergic inflammation, restoring the homeostasis of the immune system.

Chemokines is a special group of cytokines that attract leukocytes to the site of inflammation, and the immune system uses these to move leukocytes in the tissues, when they have left the bloodstream.

ALLERGIC DISEASES

RHINITIS

Symptoms frequently include sneezing, rhinorrhea, nasal itching and sneezing. Symptoms may be intermittent or persistent. Although rhinitis is not life-threatening, it can signifi cantly impact quality of life and is responsible for missed work and school days. Allergic rhinitis may contribute to fatigue, sleep disorders and learning diffi culties.

The prevalence of rhinitis is higher in males than females; this may be genetically determined, as IgE levels are higher in boys than in girls.

Classification

Allergic rhinitis

Symptoms of allergic rhinitis may include nasal congestion, repetitive sneezing, watery eyes, watery rhinorrhea and pruritus of the eyes, nose and/or throat. In allergic rhinitis there is usually a clear relationship between symptoms and exposure to known allergens, which can include pollens, dust mites, animal dander and molds.

Seasonal allergens, which include pollens from trees, grasses and weeds, as well as outdoor molds, can contribute to intermittent rhinitis, whereas perennial allergens, which include dust mites, animal dander, cockroaches and indoor molds, can contribute to persistent rhinitis. However, in tropical climates, pollens can be present throughout the year and thus cause persistent rhinitis.

Occupational agents can also cause allergic rhinitis; the nasal mucosa is easily accessible to deposition of dusts and vapors, which can be associated with IgE-mediated allergic response.

Non allergic rhinitis

Non-allergic rhinitis can be present with symptoms similar to those with allergic rhinitis, such as nasal congestion, rhinorrhea, nasal itching and sneezing, but allergy testing (skin testing or specifi c IgE blood testing) does not reveal any allergic triggers.

Environmental pollutants and occupational agents may contribute to non-allergic rhinitis.

There can be eosinophilic and non-eosinophilic subtypes of non-allergic rhinitis. With the eosinophilic subgroup, eosinophils are seen in nasal secretions when visualized by light microscopy, and the clinical presentation can be similar to allergic rhinitis, except for the absence of an identifi able allergen. The non-eosinophilic subgroup includes individuals with autonomic rhinitis where parasympathetic overdrive occurs and replaces the normal alternating sympathetic tone which leads to the nasal cycle and maintains patency. If rhinorrhea is predominantly unilateral, this may represent leakage of cerebrospinal fluid.

   

CHRONIC RHINOSINUSITIS AND NASAL POLYPOSIS

Chronic rhinosinusitis (CRS) is an inflammatory disease of the paranasal sinuses that has been present 12 weeks or longer. The four cardinal symptoms of CRS are mucopurulent drainage, nasal obstruction, facial discomfort, and decreased sense of smell; two of these must be present, along with CT or endoscopic evidence of sinus mucosal inflammation in order to consider the diagnosis. Up to one third of patients with CRS present with nasal polyps, which are likely to cause anosmia.

CONJUNCTIVITIS

Conjunctivitis refers to a large group of ocular disorders that result in conjunctival and/or corneal infl ammation. The ocular surface exhibits a variety of immunological responses due to the sensitivity of conjunctival vessels and its direct contact with the external environment.

Classification Of Allergic Conjunctivitis

Allergic conjunctivitis: is the most common form of allergic eye disease

Seasonal Allergic Conjunctivitis the most frequent form of allergic conjunctivitis. This is usually concurrent with allergic rhinitis, and usually symptoms are present bilaterally. Symptoms may include tearing and itching, burning or stinging in the eyes, bilateral ocular and periocular pruritus and milky or pinkish conjunctiva. Common airborne antigens (pollen, grass, weeds, etc.) may provoke symptoms of allergic conjunctivitis.

Perennial Allergic Conjunctivitis: may also be caused by seasonal allergens, but not exclusively; with perennial allergic conjunctivitis, there is sensitive response to allergens present year-round, and common household allergens such as dust mites, pet dander and cockroaches may play a role. This is less prevalent than seasonal allergic conjunctivitis and has milder symptoms than those associated with seasonal allergic conjunctivitis.

Atopic Keratoconjunctivitis:  Symptoms may include bilateral itching; photophobia, stringy, ropy or watery discharge; redness, burning, tearing; corneal vascularization, ulceration or scarring; papillary hypertrophy of lower and upper tarsal conjunctivae. This has a strong association with atopic dermatitis (which can affect the face and eyelids). Onset is usually late teens to early 20s, and the patient’s history of allergy is very common (especially allergic rhinitis and asthma).

Vernal Conjunctivitis: Vernal Conjunctivitis is commonly associated with personal or familial history of atopy. More than 90 percent of these patients exhibit one or more atopic conditions (especially allergic rhinitis and asthma). This usually occurs in childhood, and is most commonly seen in boys. Symptoms may include intense itching; chronic bilateral inflammation of conjunctivae; photophobia; stringy, ropy discharge; foreign body sensation; blurred vision; blepharospasm; Horner-Trantas’ dots, and giant papillae of the palpebral conjunctiva. If left untreated, scarring can lead to vision loss.

Giant Papillary Conjunctivitis is an inflammatory reaction which may be allergy involving proteins that adhere to surfaces of contact lenses, sutures, ocular prostheses and cyanoacrylate adhesives, and can lead to foreign body sensation with use. Prolonged mechanical irritation from foreign bodies is thought to be a contributing factor. Contact lenses are the most common irritant associated with giant papillary conjunctivitis. Symptoms may include ocular itching; small strand of mucus and/or stringy discharge; slight blurring of vision; mild pruritus; mild hyperemia; macro- and giant papillae on upper tarsal of conjunctivae; abnormal thickening of conjunctivae and opacification of conjunctiva.

ADULT AND PAEDIATRIC ASTHMA

Asthma is a complex heterogeneous disease characterized by chronic airway inflammation, airway hyper responsiveness, and variable airflow obstruction that reverses either spontaneously or with treatment. The hallmark respiratory symptoms that result include shortness of breath, wheezing, coughing and chest tightness; symptoms are often worse in the night or early morning.

Risk factors for developing asthma

Factors associated with asthma development, include atopy, diet, obesity, respiratory infections, non-respiratory microbial infections, premature birth, occupational exposures, air pollution (both indoor and outdoor) and tobacco smoke (both primary and secondary). Genetic linkage analysis has also identified a number of genes that have been implicated in asthma, but no direct causative effect from them has yet been proven.

The primary risk factors for developing asthma in children include atopy, parental history of asthma and viral exposure. Atopy is the single greatest risk factor for the development of asthma in infants and children. This includes both atopic dermatitis and allergies.

Asthma involves the entire respiratory tract from the upper airways to the peripheral small airways. Steroid refractory airway inflammation and remodeling is a common phenomenon in adult asthma.

ANAPHYLAXIS

The term “anaphylaxis” was fi rst proposed by Portier and Richet in 1901.1 It indicates an opposite response to that of prophylaxis-“ana” meaning “against” and “phylaxis” meaning “protection”. They accidentally discovered such a reaction opposite to their expectations when they were trying to immunize dogs against the venom of the sea anemone.

Anaphylaxis is a serious allergic or hypersensitivity reaction that can cause death. It is rapid in onset (minutes to a few hours) and typically involves multiple body systems.

The criteria for the diagnosis of anaphylaxis were established at the “Second National Institute of Allergy and Infectious Disease/Food Allergy and Anaphylaxis Network (NIAID/FAAN) symposium”.

Causes of anaphylaxis

There are a wide variety of agents that can cause anaphylaxis. Foods and medications are common causes of anaphylaxis. Almost any food or medication can produce an anaphylactic reaction.

Vulnerable patients with anaphylaxis include infants, adolescents, pregnant women, the elderly, patients with asthma, cardiovascular disease, mast cell activation disorders, and those concurrently taking certain medications. Amplifying co-factors include exercise, infection, and emotional stress.

DRUG ALLERGY

Adverse reactions to pharmaceutical and diagnostic products constitute a major hazard in the practice of medicine and are responsible for substantial morbidity and cost. Although different classifications have been proposed, adverse drug reactions (ADRs) are usefully organized into two subtypes: type A reactions, which are predictable from known pharmacologic properties, and type B reactions, which are unpredictable or unexpected syndromes restricted to a vulnerable subpopulation.

The term "drug allergy" is reserved to immunologically-mediated DHRs, after showing direct or indirect evidence of either drug-specific antibodies or T-cells.

Even though urticaria and maculopapular eruptions are the most frequent manifestations, there are many other clinical presentations artificially classified into two types, according to the delay of onset of the reaction after the last administration of the drug:

Immediate reactions, occurring less than 1 hour after the last drug intake, usually in the form of isolated urticaria, angioedema, rhinitis, conjunctivitis, broncho spasm, gastro-intestinal symptoms (nausea, vomiting, diarrhoea), or anaphylaxis with or without cardiovascular collapse (anaphylactic shock) and

Non-immediate reactions, with variable cutaneous symptoms occurring after more than 1 hour and up to several days after the last drug intake, such as late-occurring urticaria, maculopapular eruptions, fixed drug eruptions, vasculitis, blistering diseases (such as toxic epidermal necrolysis (TEN), Stevens-Johnson syndrome (SJS) and generalized bullous fixed drug eruptions), drug reaction with eosinophilia and systemic symptoms (DRESS), acute generalized exanthematous pustulosis (AGEP) and symmetrical drug-related intertriginous and flexural exanthemas. Internal organs can be affected either alone or with cutaneous symptoms and include hepatitis, renal failure, pneumonitis, anemia, neutropenia, and thrombocytopenia. The first category is mostly mediated through specific IgE, whereas the latter is specific T cell-mediated.

Mechanisms of Drug Allergy

Immune recognition

Drugs can be recognized by the immune system through a variety of pathways that can lead to immune stimulation. Generally immune stimulation requires a coordinated T and B cell response. Thus drugs must be processed by Antigen-Presenting Cells (APCs) and be recognized as foreign by T cells. Known mechanisms for such immune stimulation are as follows:

1. Hapten-carrier mechanism: Most medications are small molecules and are not recognized by immune cells or able to interact with immune receptors directly to activate the immune system. However many such small molecule drugs covalently bind larger endogenous molecules such as host proteins. Such drugs are called haptens and the complexes are referred to as hapten-carrier conjugates. These conjugates are capable of stimulating T cells and humoral immune responses. Penicillin is the most common and well-studied hapten—its beta-lactam ring readily reacts with lysine residues on proteins and forms a penicilloyl-protein and other conjugates that are readily recognized by the immune system.

2. Prohapten mechanism: Some small molecules are known to form reactive intermediates during drug metabolism. Such drug metabolites can act as haptens before they can be conjugated for detoxification. These drugs are commonly referred to as prohaptens. The most well known example are the sulfonamide antimicrobials, which during metabolism are capable of forming Sulfamethoxazole Nitroso (SMX-NO), which is highly reactive with proteins.

3. Macromolecules: There are an increasing number of medications that are macromolecules or peptides such as hormones, enzymes, vaccines and monoclonal antibodies. These can be processed by APCs and thereby activate a T cell response in their native form.

4. Direct B cell stimulation: Some small molecular weight drugs contain repetitive motifs that are adequately immunogenic to induce B cell antibody formation without requiring an antigen presenting cell or T cell help. Neuromuscular blockers are one example believed to act in this manner; they contain multivalent quaternary ammonium compounds that are adequately immunoreactive to induce drug-specific antibodies without a coordinated immune response.

5. p-i concept: An additional proposed mechanism is known as the pharmacologic interaction with immune receptors or “p-i concept”. This suggests that drugs can bind noncovalently with T-cell receptors and lead to a direct T cell response without previous sensitization. This mechanism may account for some drug reactions that occur on fi rst exposure to the agent.

6. Danger hypothesis: Another important concept in the generation of immunologic drug reactions is known as the “danger hypothesis”. This suggests that co-stimulatory signals to the immune system in the setting of infection or acute inflammation may help activate an immune system in a non-specific manner that contributes to a specific drug reaction. A common example of this is the development of a drug rash to amoxicillin in the setting of Epstein-Barr virus infection.

Immune response

Once a drug is recognized by the immune system through one of the above mechanisms, it can lead to a variety of coordinated immune responses. Traditionally these immunologic responses have been classified according to the Gell-Coombs classification as types I, II, III, or IV.

Type I reactions are responsible for urticaria and anaphylaxis. Such reactions are mediated by immunoglobulin E (IgE) directed against a drug or metabolite. Generally speaking, this type of reaction requires a sensitization stage. On initial exposure, a coordinated B cell and T helper cell response leads to B cell class switching to generate drug specific IgE molecules. These IgE molecules attach to the surface of mast cells throughout the body. Sensitization occurs without clinical symptoms. On re-exposure to the drug, the relevant drug or metabolite will cause cross-linking of the drug-specific IgE and activation of mast cells. These events cause release and production of vasoactive mediators including histamine, tryptase and arachidonic acid metabolites leading to diffuse clinical symptoms. These can range in severity and include pruritus, fl ushing, urticaria, gastrointestinal edema (causing pain, emesis or diarrhea), angioedema, wheezing, laryngeal edema and hypotension. The extreme form of such reactions is anaphylactic shock. This process usually occurs within minutes to hours after exposure and hence these are called immediate type hypersensitivity reactions. The most common example of type I hypersensitivity is an immediate reaction to penicillin antibiotics. There are also instances of immediate type hypersensitivity reactions that are not IgE mediated and are believe to be due to direct mast cell activation by a drug. These kinds of reactions are sometimes referred to as “pseudoallergic” reactions. Common examples include anaphylactoid reactions to opiates, vancomycin or iodinated contrast dye.

Type II hypersensitivity drug reactions are uncommon. They are responsible for antibody mediated cell destruction by preformed IgG or IgM. These reactions arise when drugs bind to the surface of blood cells (commonly erythrocytes or platelets) and act as haptens stimulating IgG production to that foreign hapten-protein complex. Binding of the antibody to the target cell leads to its destruction by macrophages or complement-mediated lysis. These reactions usually occur with high-dose and prolonged treatment courses. A classic example is prolonged use of the antimalarials quinine or quinidine leading to hemolytic anemia.

Type III hypersensitivity drug reactions are also uncommon and generally present as serum sickness or vasculitis reactions. They are mediated by drug specific IgG binding with that drug and causing antigen-antibody complex formation. These immune complexes can precipitate in various tissues including joints and blood vessels and lead to complement activation and inflammation. Classic symptoms are fever, pruritic rash and joint pain. This is most commonly caused by protein medications such as anti-thymocyte globulin, though can also occur with prolonged use of small molecule medications.

Type IV hypersensitivity reactions are responsible for many cutaneous drug eruptions among other reactions. Type IV drug reactions are the result of activation of T cells and do not involve antibody formation as with the other three types. They often involve the skin due to the large numbers of T cells in skin. Type IV hypersensitivity reactions are often further subdivided into subsets based on T cell recruitment of effector cells, which reflect the laboratory findings or histology of the reactions. These effector cells include monocytes , eosinophils, CD8 T cells  and neutrophils . A drug reaction can often involve significant overlap of these subdivisions. A well-known example of type IV drug hypersensitivity is allergic contact dermatitis to a topical antimicrobial such as neomycin. This reaction is akin to that seen with poison ivy, producing inflammatory plaque and/or vesicle formation.

FOOD ALLERGY

Allergies to foods associate considerable morbidity, impaired quality of life and costs – and can in some instances result in life-threatening anaphylaxis. Therefore, it is important to improve awareness and access for all to a proper diagnosis and treatment to ensure a safe and good life.

Classification

Any aberrant reaction after consumption of a food or food additive is classified as an adverse food reaction, which may be immune mediated or non-immune mediated. Immune mediated adverse food reactions are known as food allergy or food hypersensitivity and non-immune mediated adverse food reactions are called food intolerance such as lactose intolerance. Immune mediated reactions can be IgE mediated, the classical food allergy or non-IgE mediated now gaining in importance especially GI reactions.

A Food Allergy (FA) is defined as an adverse health effect arising from a specific immune response that occurs reproducibly on exposure to a given food.1 Food allergens are defi ned as those specific components of food or ingredients within food (typically proteins, but sometimes also chemical haptens) that are recognized by allergen-specific immune cells and elicit specific immunologic reactions, resulting in characteristic symptoms.

Clinical Features

Clinical symptoms of food allergy can be related to a large number of systems and can be of any severity ranging from mild self-limiting symptoms to life threatening anaphylaxis or even death. The commonest clinical symptom involves skin and is observed as urticaria and a sense of heat. In some cases, there can be angioedema of the skin and mucous membranes. The other systems involved are upper respiratory (rhinorrhoea, nasal obstruction, sneezing, nasal itching, throat itching and congestion, hoarseness of voice, stridor, itching and blocking sensation in the ears), ocular (redness, itching, watering from eyes and swelling of the eyes), lower respiratory (coughing, sputum, wheezing, breathing difficulty, respiratory arrest), cardiac (palpitations, syncope, hypotension, arrhythmias, cardiac arrest), gastro-intestinal (nausea, vomiting, abdominal cramps and diarrhoea).

Oral Allergy Syndrome

Oral allergy syndrome is seen in many subjects who have cross reactivity to various pollens that cross-react with certain foods and on consumption of the said food, the subject immediately develops itching of the mouth, palate and pharynx. Occasional throat tightness and rarely systemic symptoms can develop. Clinical symptoms are generally more pronounced after the pollen season, probably due to “priming” of the pollen specific IgE antibody response.

Non IgE mediated Food Hypersensitivity

There are several instances of food intolerant situations in early childhood, which the reader needs to be aware of in dealing with infants with food allergies.

1. FPIES (Food Protein Induced Enterocolitis Syndrome): This could be a serious issue to deal with in infants. Continual ingestion of the causal protein (milk, soy) can cause profuse vomiting, heme occult positive stools with diarrhea, failure to thrive and at times dehydration and acidosis. Skin tests to cow’s milk and soy protein are negative. Treatment includes total avoidance of milk protein, soy protein and introduction of hydrolyzed formula.

2. Dietary Protein-induced Proctocolitis: This is more of a benign condition although could be more alarming to the parents with passage of fresh red bloody mucous with normal looking stools or mucous stools. This condition can occur in exclusively breast fed infants due to maternal dietary proteins mainly cow’s milk protein. Elimination of the causal protein from the diet of the breast feeding mother or substitution with a hypoallergenic formula resolves the bleeding in 72 hours.

3. Gluten in wheat, rye and barley may elicit a non IgE mediated reaction. Gluten induced enteropathy (Celiac disease) is an immune disorder with the production of antibodies against gluten (IgA anti gliadin and anti endomysial antibodies) causing steatorrhoea, flatulence and weight loss. Biopsy of the jejunum exhibits flattening of the villi.

IgE and Non IgE Mediated Food Hypersensitivities

1. Atopic Dermatitis (AD): around 38 percent of children with AD have exhibited IgE mediated food allergies. The more severe the dermatitis, the more prevalent is the food allergy in these children. Dietary elimination ameliorates rash as well as the severity of the disease. Skin testing as well as Immunocap studies support IgE mediated food sensitivities in these patients. The major food items (in 85 percent of cases) include cow’s milk, egg, soy, peanut, tree nut, fish and shell fi sh.

2. Eosinophilic oesophagitis: This is a syndrome, fairly new to the allergist’s community, of severe reflux symptoms which are resistant to the standard therapy including proton pump inhibitors. Generally younger adults are affected although this is also seen in paediatric age groups. Generally symptoms include a sensation of “food getting stuck”, abdominal pain, early satiety, vomiting and diarrhoea at times. The hall mark of the diagnosis is the presence of eosinophilic infiltration (> 15 eosinophils/HPF) in the oesophageal biopsy specimen. Location of the inflammation determines the symptoms: dysphagia due to oesophagitis and obstruction due to gastric outlet infiltration.

3. Food dyes and additives: Adverse reactions to food dyes, additives and certain preservatives are generally rare (<1%). Direct challenge techniques are used for diagnostic purposes. Sulfite sensitivity is a more recognized phenomenon. Sulfites (Sodium Meta bisulphate) a commonly used preservative in food and drug industry has been implicated in certain types of asthma. 

ATOPIC DERMATITIS

Atopic dermatitis is a chronic inflammatory skin disease with a pathogenesis of complex immune dysregulation and interplay of genetic, epidermal and psychological factors. The stratum corneum of healthy skin functions as a barrier and provides water-retaining properties. It contains an extracellular lipid matrix including ceramides, cholesterol and free fatty acids. When this layer becomes dry, and fissured, it becomes a portal of entry for bacteria such as Staphylococcal aureus. Disruption of the integrity of the stratum corneum exposes epidermal and dermal extracellular matrix proteins, such as fibronectin and collagen, which can serve as anchors for S. aureus binding via adhesins. In AD, the stratum corneum lipid composition contains decreased levels of ceramides and sphingosine which normally act as water-retaining molecules. Deficient ceramide increases secretion of ceramidases, which leads to increased transepidermal water loss, resulting in dry, cracked skin of AD. Sphingosine has been shown to normally possess antimicrobial properties, thus deficiencies may favour bacterial colonization.

Superantigen colonization results from defective skin barrier function, increased S. aureus adhesion to AD skin, and decreased innate immune responses resulting in failure to inhibit growth of S. aureus in the skin.The skin of patients with AD exhibits a striking susceptibility to colonization with Staphylococcus aureus.

This is in contrast to the skin flora of nonatopic patients in whom the colonization rate of S. aureus is less than 5 percent. Some strains of S. aureus secrete exotoxins with T-cell superantigen activity (toxigenic strains). Such bacterial superantigens can lead to activation and proliferation of T cells in AD skin, releasing cytokines and other inflammatory mediators, and contribute to the pathogenesis and exacerbation of AD.

Clinical Features

AD typically appears in early childhood before 5 yr of age and the diagnosis is based on characteristic signs and symptoms. The major features include intense pruritus, facial and extensor involvement in infants and children, flexural lichenification in adults, a chronic and relapsing course, and a personal or family history of atopic disease. Minor features include: xerosis, cutaneous infections, non-specifi c dermatitis of the hands or feet, ichthyosis, palmar hyperlinearity, keratosis pilaris, pityriasis alba, nipple eczema, white dermatographism and delayed blanch response, anterior subcapsular cataracts, elevated serum IgE levels and positive immediate type allergy skin tests. Physical examination findings during infancy include involvement of the face, scalp and extensor surfaces of the extremities, with sparing of the diaper area. In older patients, the flexural folds of extremities are predominantly involved. Acute AD lesions are intensely pruritic, erythematous papules associated with excoriation and serous exudation. Chronic lesions have undergone tissue remodelling from chronic inflammation and are associated with thickened plaque and increased skin markings (lichenification), and dry fibrotic papules.

URTICARIA

Urticaria (hives) are lesions that are typically edematous with pale raised round or oval centers (wheals), of varying size and may occur anywhere on the body. The area surrounding the wheal is commonly erythematous but well circumscribed and blanches with pressure. Urticaria may coalesce and thus create an appearance of irregular margins, but remain well defi ned. The lesions are transient in duration, lasting minutes to days and resolve without inherent scar or dyspigmentation. Typical lesions are markedly pruritic and are not associated with pain or burning sensation. Atypical urticaria lesions can be nonblanching, burning in character, last more than 24 hours and leave residual dyspigmentation or scar formation. These may represent vasculitic disease and warrant further evaluation.

The major effector cells of urticaria are mast cells although basophils are also implicated. Mast cells are found in the superficial dermis and sub-dermis in proximity of blood vessels as well on in mucosal surfaces of the mouth, nose, lungs and digestive tract. The allergic response of immediate hypersensitivity occurs when allergens cross-link immunoglobulin E (IgE) that is bound to high-affi nity IgE receptors (FcεRI) on the surface of mast cells which then leads to mast cell activation. However, mast cell activation may occur by IgE-independent means and this has been termed a pseudoallergic response although this term is being used less often as specific receptors and pathways are identified. This phenomenon has been observed with physical stimuli such as heat or pressure, with medications such as NSAIDs, opioids and vancomycin, as well as with radiocontrast dye. Viral infections may also lead to urticaria that is likely not IgE-mediated.

Furthermore, a toxic reaction due to the ingestion of fish contaminated with histamine producing bacteria, scombroid food poisoning, results in hives.

Causes of Urticaria

Acute Urticaria. The causes of acute urticaria include ingestant or contact exposure, toxic reactions and pseudoallergic reactions. Often a cause of acute urticaria is unclear and these cases are deemed to be idiopathic. Episodes of acute urticaria are commonly due to allergic reactions that are IgE-triggered mast cell responses and can also occur frequently during or following an inflammatory process such as viral illness as frequently observed in children. The most frequent IgE-mediated allergic reactions are to medications or foods although some medications activate mast cells through IgE-independent pathways. The most frequently implicated medications are diuretics, muscle relaxants, nonsteroidal anti-inflammatory drugs (NSAIDs), penicillins and sulfonamides. Acute urticaria related to foods has been reported in increasing frequency in developed countries. The most common culprits in children are milk, egg(s) and peanuts, whereas in adults, peanut(s), tree nuts, finfish and shellfish are most common.

Chronic urticaria.  75 percent of cases of chronic urticaria are classified as idiopathic. Patients with chronic urticaria often experience symptoms without specific physical triggers, allergen exposure or comorbid illness. However, in half of the above cases (40 percent of total cases), evidence of autoimmunity such as the presence of immunoglobulin G (IgG) antibodies that can cross-link FcεRI and/or bind to thyroperoxidase or thyroglobulin can be demonstrated. Whether this autoimmune subgroup should be separated from the idiopathic group is debated.

Physical stimuli activate mast cells by unknown mechanisms. Physical urticaria/ angioedema accounts for approximately 20 percent of chronic urticaria cases. Many physical stimuli have been reported including exercise, hot temperature, cold temperature, solar radiation, water, pressure and vibration. The most common physical urticaria, affecting 2 to 5 percent of the population, is dermographism (also called dermatographism) which leads to acute wheal production by scratching or stroking of the skin.

Cholinergic urticaria is the most common single cause of chronic urticaria accounting for up to 15 percent of cases. The mechanism is related to cholinergic stimulation of mast cells after exposure to exercise or heat or by an increase in basal body temperature. Lesions are atypical from conventional hives in that they are pale-centered and punctate (0.1 to 1 cm in diameter) in appearance although they may coalesce into larger lesions.

Cold urticaria is characterized by quickly occurring erythema, pruritus and oedema following exposure to the cold. Lesions develop on cold-stimulus exposure body areas and symptoms often peak once the cooled area is warmed. Thus, cold urticaria can have localized findings with exposure to cold wind on the face, consuming cold foods or holding cold objects such as ice cubes. But symptoms can also be diffuse and potentially fatal as with cold water exposure while swimming. Swimming in cold water results in a large amount of mast cell activation and can lead to the cholinergic symptoms as above as well as hypotension that can be fatal due to resultant syncope and drowning; patients should be informed of the increased risk of death with swimming in cold water.

Solar urticaria is a rare disorder that occurs after minutes of exposure to sunlight. IgE to a photo-induced allergen may be important in some patients. Pruritis, erythema and swelling occur and are confined to light-exposed areas. However, with exposure of large body surface area, systemic symptoms such as bronchoconstriction, hypotension and even death may occur.

ANGIOEDEMA

Angioedema may occur without associated urticaria and is distinct from angioedema associated with urticaria. Patients with isolated angioedema fall into two broad categories. The first group includes hereditary angioedema (HAE), acquired angioedema (AAE), and angiotensin converting enzyme (ACE)-associated angioedema. In these diseases the angioedema is due to activation of the kinin system. The second group, idiopathic angioedema (at one time called histaminergic angioedema), appears to be mechanistically related to urticaria/angioedema in that these patients are responsive or at least partially responsive to antihistamines and corticosteroids. In this setting, the angioedema is likely due to mast cell activation.

Pathogenesis

During episodes of angioedema of any cause, vasoactive mediators lead to vasodilation and increased vascular permeability resulting in plasma leakage. However, this occurs in deeper tissue layers including the deep dermis and subcutaneous tissues. Symptoms range from discomfort to the sensation of severe pressure, and in the case of laryngeal oedema, angioedema can be fatal.

Hereditary angioedema type I and II are caused by deficient or defective C1 inhibitor, a regulatory protein which is involved in controlling the complement and kinin-generating pathways. C1 inhibitor inhibits proteins that promote bradykinin generation and activation of the classic complement cascade. Excess bradykinin, a vasoactive peptide, leads to plasma leakage and resultant angioedema.

CONTACT DERMATITIS

Contact Dermatitis (CD) is a common skin disorder marked by erythematous, vesicular, papular or lichenified pruritic skin lesions. It is caused by direct contact of an agent to the skin. It can be caused either by irritant triggers in 80 percent of patients versus allergic triggers in the remaining 20 percent of patients.

Allergic Contact Dermatitis (ACD) is a cutaneous reaction caused by a type IV cell-mediated reaction to an allergen; the reaction will recur with subsequent re-exposure to the same or cross-reacting allergen. All age groups are affected by CD even though it is rarer in the first 10 yrs. of life. There is a slight female preponderance that may be due to greater exposure to cosmetics and jewellery. The allergens in ACD in children are generally comparable to the general adult population, with similar occurrences of nickel, fragrances, Toxicodendron and rubber chemicals

Irritant Contact Dermatitis (ICD) is caused by non-immunologic direct tissue activation T-cells are stimulated to release inflammatory Th1 cytokines such as TNF-α, IL-1, IL-8 an GM-SCF via non-immune mechanisms, thus no immunologic memory or sensitization period is necessary for these reactions. In these irritant reactions there tends to be a higher concentration of the agent needed to provoke a response; it may also produce a burning and stinging sensation in the patient as opposed to an itch. The area of involvement is usually limited to the area that comes into contact with the involved agent and is a dose dependent response. Common agents that produce ICD include water, soap, detergents, acids/bases and bodily fluids including urine, saliva and stool.

LATEX ALLERGY

Latex allergy emerged as a major public health problem in the 1980’s from the widespread use of latex gloves in the healthcare setting to protect against the transmission of viral infections, including hepatitis and human immunodeficiency virus, on a background of a rising prevalence of atopic disease. The presentation of IgE-mediated Type I latex allergy ranges from mild itching and erythema at the site of skin or mucosal contact, localised urticaria to anaphylaxis. This diverse spectrum of reactions has been referred to as the contact urticaria syndrome.

Individuals at risk of immediate hypersensitivity reactions to latex include health care workers, dentists, patients with spina bifida requiring long term catheterisation, those with atopy or hand dermatitis and workers in the rubber industry. Wearing powdered gloves appears to increase the risk of allergic sensitisation substantially and these have been banned in most healthcare settings in favour of non-powdered gloves and non-rubber alternatives, including vinyl and nitrile.

Developing allergic contact dermatitis to chemicals used in the manufacture of rubber products (type IV hypersensitivity), including rubber accelerators, and the risk of irritant contact dermatitis through prolonged glove wearing remains a problem in some occupations, including dentistry, hairdressing, mechanics and manufacturing industry.

Natural rubber latex is harvested from the rubber tree, Hevea braziliensis, which is indigenous to the Amazon region, but mainly cultivated in Malaysia and Thailand. Latex forms the cytoplasm of specialized cells called lacticifers that function to seal damaged sites.

Immediate type I hypersensitivity to latex is mediated by an IgE response to these latex proteins. Exposure can occur via skin, mucous membrane or the respiratory tract. In addition, mucous membranes of the gastrointestinal and urogenital tracts can be exposed by direct contact with NRL catheters and internal exposure can occur during surgical procedures by the use of NRL gloves or internally placed latex materials such as wound drains.  Patients become sensitized with plasma cells producing IgE to latex proteins. The IgE binds to the surfaces of mast cells and upon re-exposure to latex these sensitized cells release mediators leading to the onset of symptoms.

Type IV allergy to latex is a delayed cell-mediated reaction where allergen interacts with Langerhans cells in the skin and is presented to CD4+ T cells inducing the clonal expansion of antigen-specific T lymphocytes.19 Further clonal expansion occurs in the regional lymph nodes and the sensitized T cells are then dispersed throughout the body. Upon subsequent exposure of the skin to sufficient antigen, a local inflammatory response ensues. This occurs over 24–48 hr and results in a pruritic, eczematous reaction at the site where allergen has been in contact with the skin.

INSECT STING/VENOM ALLERGY

There are three families of the Hymenoptera order of insects that are predominantly responsible for venom allergy in humans:

Apidae (honey bees and bumblebees)

Vespidae (yellow jackets, hornets, paper wasps)

Formicidae (fire ants)

Clinical symptoms following a sting are highly variable and difficult to predict. Reactions are  classified as local or systemic.

Local reactions of erythema, oedema, pruritus and induration are limited to the are surrounding the sting site and if >10 cm is termed a large local reaction (LLR). Large local reactions are not a risk factor for a SR to future stings, hence avoidance of the stings would be crucial in the management. Antihistamines and occasional use of oral corticosteroids reduce the intensity of the reaction but prescription of an epinephrine auto-injector is not usually recommended.

Systemic reactions vary from mild cutaneous reactions to severe life threatening anaphylaxis. These are IgE mediated resulting in mast cell degranulation and the release of histamine and other vasoactive and pro-inflammatory mediators.

OCCUPATIONAL ALLERGY

Occupational allergy refers to those disorders or conditions that are caused by exposure to substances in the work environment and in whose pathogenesis allergic factors are determinant. The allergic diseases that may be contracted as a consequence of exposure to sensitizing agents in the workplace are rhinitis, conjunctivitis, asthma, eosinophilic bronchitis, hypersensitivity pneumonitis, allergic contact dermatitis, immunologic contact urticaria and occupational anaphylaxis.

Occupational rhinitis is an inflammatory disease of the nose, characterized by intermittent or persistent nasal symptoms, and/ or variable nasal airflow limitation and/or hypersecretion due to causes and conditions attributable to a particular work environment and not to stimuli encountered outside the workplace.

Occupational asthma (OA) refers to de novo asthma or the recurrence of previously quiescent asthma induced by either sensitization to a specific substance (eg, an inhaled protein or a chemical at work), which is termed allergic or sensitizer-induced OA, or by exposure to an inhaled irritant at work, which is termed irritant-induced OA. These agents are categorized into high-molecular- weight (HMW) compounds, which are proteins acting through an IgE-mediated mechanism, and low-molecular-weight (LMW) compounds, which are chemical sensitizers that, with few exceptions, are not associated with an IgE-dependent mechanism.

Hypersensitivity pneumonitis (HP) is an allergic lung disease that occurs as the result of an immunologic inflammatory reaction to the inhalation of any of a variety of organic dusts or LMW chemicals with or without systemic manifestations. The disease is a diffuse, predominantly mononuclear inflammation of the lung parenchyma, particularly the terminal bronchioles, interstitium, and alveoli.

Occupational allergic contact dermatitis is one of the most common occupational diseases.

ALLERGIC BRONCHOPULMONARY ASPERGILLOSIS

Allergic bronchopulmonary mycoses are rare pulmonary hypersensitivity reaction, most commonly directed against Aspergillus fumigatus (Allergic Bronchopulmonary Aspergillosis - ABPA). Many other fungi can also serve as antigen source, hence the term allergic bronchopulmonary mycosis.

ABPA occurs on the background of either allergic asthma or cystic fibrosis.  The pathogenesis of ABPA involves complex local as well as systemic hypersensitivity reactions to fungal antigens from fungal mycelia, which grow in the bronchial lumen, where they cause local inflammation and subsequent tissue destruction resulting in central bronchiectasis. In addition to a marked cellular inflammation with eosinophils and neutrophils there is a humoral component involving polyclonal IgE-, IgG- and IgA production. Consequently, the complex immunopathogenesis of ABPA appears to involve type I as well as type III immune reactions. Comorbidities such as allergic rhinitis are frequent.

Symptoms include wheezing, cough, malaise, fever, chest pain and the production of copious amounts of purulent sputum, which often contains fungal hyphae. Radiological features of ABPA are central bronchiectasis and fleeting pulmonary infiltrates, which do not respond to antibiotic therapy but to systemic corticosteroids instead. Most patients have a considerable airflow obstruction, often combined with loss of lung volume due to pulmonary destruction by infiltrates and bronchiectasis and require high dose bronchodilator therapy in addition to anti-inflammatory glucocorticosteroids.

ALLERGY DIAGNOSIS

INVIVO ALLERGY DIAGNOSIS

SKIN TEST

Allergy Skin testing (AST) is a well-established diagnostic procedure which has been in vogue for more than 100 years. It has been described by Charles Harrison Blackley1 in the 1860s in his research studies on “hay fever”.

AST is a bioassay that facilitates the exposure to the antigen and indicates the presence of specific IgE antibodies on the surface of the sensitized mast cells. AST develops a wheal and fl are a wheal and flare developing over a period of 15–20 min following the introduction of the allergen to which the subject is sensitized. Sensitization entails the presence of specific IgE antibodies on the surface of the mast cells. “Sensitization” is an immunologic term; it does not mean the person is “allergic” which a clinical term. AST detects only sensitization, and correlation of the AST with clinical history will determine allergy.

AST is used for several purposes, the most common being the pivotal role in the diagnosis of IgE mediated clinical conditions such as allergic rhinitis, allergic conjunctivitis, allergic asthma, atopic dermatitis, food allergy and drug allergy. AST is also utilized for standardization of allergenic extracts, epidemiological studies, pharmacological studies, as well as to evaluate the effect of anti-allergic treatment.

Mechanism of allergic reaction

There are two phases in allergic reaction: 1. Sensitization 2. Re-exposure.

Sensitization: In genetically susceptible individuals, initial exposure to the antigen (allergen) will initiate activation of the T cell through its presentation by the antigen presenting cell (APC). The differentiated T cell in turn causes class switching of the B cell through IL-4 and IL-13 to produce IgE. IgE that is released will circulate in the blood as Free IgE as well as attach to the high affinity FCeRI receptor on the surface of mast cells and basophils. This phase, where the specific IgE are bound to the mast cells, is called Sensitization.

During AST the mast cell is exposed to the antigen. If the mast cells are sensitized, cross linking of the cell bound IgE by the antigen and the release of various mediators (histamine, tryptase, leukotrienes) occur. These mediators cause vasodilatation, increased vascular permeability and exudation, which cohesively result in wheal-and-flare reaction.

There are two types of AST:

1. Epicutaneous test: Prick, Puncture, Scratch.
2. Intradermal skin test (IDST)

Technique of skin testing

The basic idea of AST in general, is to introduce a very small amount of the antigen into the dermis through the surface of the stratum corneum. The mast cells are located in the dermis and on specific interaction of the antigen with the specific IgE, the mediators will be released to cause the wheal and flare reaction.

1. Epicutaneous ST

Scratch ST: A superficial abrasion is made with a needle or any other sharp instrument, and the antigen is applied to the site.

Puncture ST: The antigen is placed on the skin first, and a superficial break in the epidermis is made by a perpendicular pressure through the drop, usually with a needle.

Prick ST: A drop of the antigen is placed on the skin and a sharp instrument is passed through the drop, penetrating the skin at approximately a 45-degree angle. The device is then lifted, creating a small break in the epidermis. It is estimated that only 0.3 microliters of fluid is introduced into the skin.

Of the three types of epicutaneous ST, the prick ST has been the most popular and most commonly used due to its safety, reliability and sensitivity.

Common errors in prick testing

• Tests are placed too close together (<2 cm), and overlapping reactions cannot be separated visually.
• Induction of bleeding can lead to false-positive results.
• Insufficient penetration of skin by puncture instrument can lead to false-negative results; this occurs more frequently with plastic devices.
• Allergen solutions can spread during the test or when the solution is wiped away.

2. Intradermal skin test (IDST)

The test is performed using 25–27-gauge needles and allergist syringes. A small amount of antigen (0.02–0.05 ml) is injected into the skin to raise a small wheal 2–3 mm in diameter. The space between test sites should be a minimum of 2cms. Concentration used for ID skin testing is generally 1:1000 or 1: 500 fold more dilute than the concentration used for epicutaneous ST. There are several variables associated with ID ST such as the depth of the injection, amount injected, as well as any bleeding at the site of injection. These factors could potentially affect the results of the IDST. IDST is riskier than epicutaneous ST and adverse reactions including anaphylaxis have been reported. Hence extreme care should be undertaken in performing these ST and emergency tray cart with trained medical personnel and the physician’s physical presence are mandatory.

Common errors in intradermal skin testing

• Test sites are too close together, and false-positive results can be observed.
• Volume injected is too large (>0.1 mL).
• High concentration of allergen can lead to false-positive results.
• Splash reaction is caused by air injection.
• Subcutaneous injection leads to a false-negative test (i.e., no bleb formed).
• Intracutaneous bleeding site is read as a positive test result.
• Too many tests performed at the same time may induce systemic reactions.

Positive & Negative Controls

It is very important to perform and interpret the control site reactions. Negative saline control is interpreted as acceptable between 0–3 mm in diameter. Positive histamine control has to be 3 mm larger than the negative saline control. A positive ST is interpreted as 3 mm or > higher than the negative saline control. The negative control signifies the baseline reactivity of the skin. A histamine positive control less then 3 mm in diameter suggests interference such as suppression by antihistamines.

Skin testing precautions

• Never perform skin tests unless a physician is immediately available to treat systemic reactions.
• Have emergency equipment, including epinephrine, readily available.
• Be careful with patients with current allergic symptoms.
• Determine the potency and stability of the allergen extracts used.
• Be certain that the test concentrations are appropriate.
• Include a positive and a negative control solution.
• Perform tests in normal skin.
• Evaluate the patient for dermographism.
• Determine and record medications taken by the patient and time of last dose.
• Record the reactions at the proper time.

Factors influencing Allergen Skin testing

1. Medications: Significant suppression of the skin reactivity is seen following conventional doses of antihistamines. It is important to instruct patients to stay off all antihistamine preparations for a minimum of 3 days prior to the AST. It is very important to routinely double check with the patient or patient’s family that these instructions are being followed.

Antihistamines: H1 blockers: Cetirizine, Loratidine, and Terfanidine suppress AST reactivity for 3–5 days.

Astemizole suppresses AST reactivity for 60 days.

H2 agents: no effect on AST reactivity.

Ketotifen: suppresses AST reactivity for 5 days.

Tricyclic anti-depressants: suppress AST reactivity for 2 weeks.

Steroids: No effect on AST reactivity.

2. Age: The skin reactivity generally declines with age, especially after 60 yr of age. Reaction to histamine is generally smaller in infants.

3. Area of the body: The mid and upper back are more reactive (33 percent) than the lower back. The back as a whole is more reactive (53 percent) than the forearm. The ante cubital area is the more reactive in the upper arm and the wrist is the least reactive. The radial side of the forearm is less reactive (50 percent) than the ulnar side.

4. There has to be a minimum distance of 2 cm between any two contiguous test sites. This is to prevent a non-specific enhancement through the axon reflex from a nearby strong reaction.

NASAL PROVOCATION TESTING

Inhaled allergens are an important cause of allergic rhinitis (AR) and asthma. Allergen inhalation challenge mimics the natural situation and is useful for understanding the mechanisms of allergic airway inflammation and airway hyperresponsiveness (AHR). Moreover, provocation tests play an important role in the diagnosis of asthma, and to a lesser extent also in AR. Via a bronchial provocation test (BPT), AHR can be measured by challenging the airways with a variety of physical and inhaled chemical stimuli resulting in airway constriction. The airway narrowing is measured by changes in FEV1 after gradually increasing the dose of provoking agent.

A nasal provocation test (NPT) is used in case of a typical history of AR and a negative allergy test. It is of importance to differentiate AR from non-allergic rhinitis since management may be different. A positive test result is defined by the presence of symptoms typical for AR plus objective parameters, such as decreased upper airway patency (measured with acoustic rhinometry or anterior rhinomanometry) or nasal secretion of inflammatory mediators. Patients can have immediate or dual responses.

Methods

Different methods for NPT and measurement of nasal responses are used. Medication that may interfere with the results should be stopped prior to the test and contra-indications should be ruled out.

Inhaled allergen challenge starts early in the morning in order to measure a late asthmatic response. Ideally, patients are observed for a longer period. It is strongly recommended to perform BPT in a clinical setting. In daily clinical practice for AHR diagnosis, the preferred agents are histamine, methacholine and mannitol. These agents give a bronchoconstrive response that is less prolonged and quickly reversible with bronchodilators and inhaled corticosteroids.

Limitations

Inhaled allergen provocation tests are safe in a dedicated setting with experienced staff. Several objective and validated parameters to measure response are available for BPT, but not for NPT. Allergen application may produce nonspecific reactions. A limited number of standardized allergen extracts are commercially available. Allergen challenge can result in a transient increase in symptoms and medication use.

Safety and side effects

NPT and BPT are safe procedures. Generalized reactions are rare after BPT, but asthma exacerbations

may occur. After allergen challenge the magnitude of the bronchoconstriction may be more difficult to control than with direct challenges, such as histamine or methacholine. Moreover, the induced bronchoconstriction is usually prolonged due to the release of proinflammatory mediators. Therefore, it is necessary that during an inhaled allergen provocation a doctor is present, the rescue medication is ready to use and a resuscitation set is available on the ward.

FOOD PROVOCATION TEST

Food challenge might be done in patients with vague symptoms possibly related to food allergy, with a significant proportion of patients having positive IgE tests not related to the symptoms but to an atopic predisposition (false positive tests indicating only sensitization). In this context, a food challenge will be the gold standard for diagnosis.

Several variables have to be taken into account when organizing a food challenge. Most importantly, the food challenge must be designed in a safe way. The food challenge must be performed in a location, where emergency medication can be administered. When doing high risk challenges, it is important to have rapid access to in-tensive care facilities. In all cases, the food challenge must be supervised by nurses trained to recognize early signs of reactions. Also, the supervising physician must be trained in order to have sufficient experience in interpreting the various clinical signs possibly observed during a food challenge.

The food is provided to the patient in increasing doses. The starting dose, as well as the time interval between the doses, will be determined by the initial history of the patient and by the purpose of the challenge. A food challenge aiming to determine the threshold level of a reaction will start with a very low dose.

The food can be administered openly, but also in a double-blind placebo-controlled manner. During this procedure, the challenge is separated into two parts, the food being hidden in a vehicle in order for blind both the patient and the examiner.

DRUG PROVOCATION TEST

A drug hypersensitivity (DH) reaction is particularly difficult to prove, as the history may be unreliable and for many drugs and reactions no sensitization can be demonstrated by skin tests or in-vitro tests. A drug provocation test (DPT) is performed:

a) to exclude DH in a non-suggestive history (e.g., unspecific symptoms arising after intake of a penicillin),

b) if a cross-reactivity in proven hypersensitivity has to be excluded (e.g., to other pain medications in acetylsalicylic acid hypersensitivity),

c) if a firm diagnosis in suggestive history of DH with negative, non-conclusive or non-available allergological tests shall be established. 

IN VITRO LABORATORY TESTS FOR THE DIAGNOSIS OF ALLERGY

In the diagnosis of human allergic disease in vitro laboratory tests for total IgE and specific IgE antibodies are often used when a patient’s clinical history identifies atopy and a relevant allergen exposure.

1. MEASUREMENT OF TOTAL IgE

The immunoglobulin, IgE, is associated with type I hypersensitivity and is an important component in allergic disorders.3 IgE is the least abundant isotype in the blood of normal, non-atopic individuals with typical levels of only 0.05 percent of the total immunoglobulin concentration. However, this low amount is capable of triggering powerful allergic reactions. Atopic individuals can have levels that far exceed those found in normal individuals. 

IgE was discovered in 1966 by Ishizakas and Hornbrook  IgE can specifically recognize allergens that are typically proteins, e.g., the dust mite DerP1, cat Feld 1 or components from grass, weed, tree pollens and certain foods. The symptoms of allergy are the result of the release of inflammatory mediators from basophils and mast cells that have IgE antibodies bound to their high-affinity receptor, FcεR1. The crosslinking of specific IgE molecules by an allergen causes the release of chemicals such as histamine, leukotrienes, and cytokines that result in many of the conditions seen in patients with allergy, asthma, eczema and allergic rhinitis.

Since the level of total serum IgE in individuals is lower than the other immunoglobulins, sensitive immunoassays have been applied to its measurement. Past methods which are no longer commonly used involve competitive-binding liquid-phase immunoprecipitation, competitive-binding solid-phase labelled-antigen immunoassays and non-competitive solid phase labelled-antigen immunoassays. The one which is most commonly used in clinical and research laboratories is the non-competitive solid-phase two-site immunoassay. As originally described, the assay employed a polyclonal anti-human IgE covalently bound to a solid support, e.g., a cellulose disk. After incubation with the patient’s serum, followed by a buffer wash to remove unbound serum components, the amount of bound human IgE to the disk is determined by the addition of a radiolabelled anti-human IgE. The results are then compared to a calibrator or control. Thus the amount of bound radiolabelled anti-IgE is directly related to the amount of IgE present in the patient’s serum. During the latter part of the 20th century manufacturers of instruments and reagents developed non-radioactive immunoassays that replaced the older radiolabelled reagents. In the United States, the Food and Drug Administration (FDA) has approved several immunoassay methods for the measurement of total IgE using non-radioisotope labeled anti-IgE. Among these are the Thermo Fisher Phadia ImmunoCAP (Uppsala, Sweden), the Siemens Immunolite (Tarrytown, NY) and the Hycor Hytec (Indianapolis, IN) systems.

2. MEASUREMENT OF SPECIFIC IgE

Shortly after the discovery of IgE, the radioallergosorbent test (RAST) was introduced for the detection of allergen specifi c IgE in the patient’s serum. As originally described this non-competitive, solid phase assay consisted of incubating a patient’s serum with an allergen that is conjugated to a cellulose paper disk that has been activated with cyanogen bromide. If allergen-specific IgE is present, it binds to the paper disk and unbound antibodies are washed away after the incubation period. IgE bound to the allergen is then detected by the addition of a radiolabelled (typically ¹²⁵I) antihuman IgE directed toward the Fc region. The amount of bound radiolabelled antihuman IgE is measured in a gamma counter and the radioactivity measured is proportional to the amount of allergen specific IgE present in the initial serum sample.

A number of changes in the RAST assay have evolved over the past 45 yr including, most notably, the use of enzyme conjugated antihuman IgE antibody in place of the radiolabelled antibody.

With the improvements in allergen extraction techniques and identification of relevant allergens, the number of allergens available has increased substantially since the assay was first employed. The use of cellulose as the binding matrix for allergens is still employed by some companies (Hycor), however others have used a cellulose sponge matrix (ImmunoCAP, Thermo Fisher Phadia) or biotinylated allergens (Siemens Immulite) to increase the allergen binding capacity. Automation of the process has also allowed for the clinical laboratory to run multiple allergens on multiple samples with rapid throughput. These changes have resulted in assay that have increased sensitivity, specificity and reproducibility compared to the earlier assays.

Testing for specific IgE initially used crude allergen extracts that contained both allergenic and non-allergenic molecules. Some of these molecules may share structural features with other environmental allergens. For example, individuals who are positive for IgE to birch pollen may also show elevated IgE to peanut and hazelnut. Using crude allergens to detect specific IgE, therefore, has the potential to lead to falsely elevated specific IgE. However, recent progress in molecular biological and biochemical techniques has now made possible the production of recombinant allergens as well as individual allergenic proteins for a given allergen. For instance, Ara h 1 through 8 are individual allergenic components of the peanut allergen. It is now possible to use these allergen components in “Component Resolved Diagnostics” or CRD tests to more accurately diagnosis allergy. In addition, studies are being conducted to determine if specific components of the allergen correlate better with true allergy and whether reactivity to individual components have a role in predicting the progress or resolution of allergy.

3. BASOPHIL ACTIVATION TEST (BAT) 

Basophils present in the peripheral blood generally represent less than 1 percent of the total leukocyte population. The granules of basophils contain histamine, proteoglycans, proteolytic enzymes, leukotrienes and several cytokines including IL-4. Basophils share many properties with tissue mast cells especially the histamine content and the presence on their surface of FcεR1.

Basophils can be activated by one of four different mechanisms:

• cross-linking of the FcεR1 via an allergen-IgE interaction,
• cross-linking with specific anti- IgE or anti-FcεR1 antibodies,
• by lectins that cross-link by an antigen-independent manner or
• independently of FcεR1 cross-linking via different receptors.

Historically basophil activation has been measured by the release of cysteinyl leukotriene or histamine. Recently investigators have used flow cytometry along with specific markers to examine basophil activation. These include CD63 which is present on the intracellular granules membrane and thus not detected in inactivated basophils by surface staining with an anti-CD63 antibody. However, upon activation and exocytosis the granules fuse with the cell membrane resulting in CD63 expression on the cell surface in a relatively high density.23 CD203c is a lineage specific marker which is constitutively expressed on the surface of basophil membranes and upon activation it is rapidly upregulated. Antibodies to the FcεR1 present in the serum of patients with chronic urticaria have been detected using CD203c as a marker of activation. The usefulness of in vitro BAT for the detection of allergen-induced expression of CD63 or the upregulation of CD203c has been demonstrated for various IgE-mediated allergies and is being investigated for its utility as a diagnostic tool.