Assessment of Allergenicity

Biochemical methods for analyzing allergens, such as protein composition and concentration, are practical but tell nothing about the allergenicity of the extract. Immunologic reactivity with IgE antibodies as assessed in vitro and in vivo provide this information. Preparations of inhalant allergens contain more than one antigen. Of the several antigens in a mixture, usually one or more dominate in both frequency and intensity of skin reactions in sensitive persons. It is inferred from this that these antigens are the most important clinically. Not all persons allergic to a certain pollen allergen react to the same antigens from that pollen allergen extract, however. The antigens of tree, grass, and weed pollens are immunologically distinct, and this agrees with the clinical and skin test data. As more allergens are isolated and purified, it is hoped that correlations between immunogenicity and biochemical structure will emerge.

Marsh (12) proposed that a major allergen be designated when 90% of clinically allergic persons react by skin test to a concentration of 0.001 mg/mL or less of that particular extractable allergen. Others suggest that a component that binds IgE in 50% or more of sensitive patient sera tested by radioimmunoelectrophoresis (another immunologic assay) should be considered a major allergen (34). This definition currently is widely accepted. A minor allergen would be one that does not meet either of these criteria.

Naturally occurring atopic allergens have few physiochemical characteristics to distinguish them from other antigens. All are proteins or glycoproteins, although high molecular weight polysaccharides that react with IgE have been obtained from Candida albicans. Most protein allergens that have been identified are acidic, with molecular weights ranging from 5,000 to 60,000 daltons. It has been postulated that larger molecules cannot readily penetrate the mucous membranes. Highly reactive allergens of lower molecular weight are described in conjunction with ragweed and grass pollens. The antigenic determinants that react with IgE antibody molecules have not been clearly identified for most allergens, although it is postulated that there must be at least two such groups on each allergen molecule to trigger the allergic response. The sequence of amino acids in some determinant groups, with less regard for conformation of the protein molecule, is most important for the major codfish antigen Gad c 1 (codfish antigen M) (35). In other allergens, such as ragweed Amb a 3 (Ra 3) and Amb a 5 (Ra 5), the conformation of the native protein is critical for allergenicity (36).

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