Isolation and characterization of the 18‐kDa major apple allergen and comparison with the major birch pollen allergen (Bet v I)

The major allergen from birch pollen, Bet v I, and the cross‐reacting 18‐kDa major allergen from Golden Delicious and Granny Smith apples were isolated by micropreparative SDS‐PAGE followed by electroelution. In the case of apples, highly active, low‐temperature extracts were used. The purity of the allergens was checked by analytic SDS‐PAGE and immunoblotting with allergic patients’ sera, as well as by N‐terminal amino acid microsequencing, and the allergens were found to be very pure. The strong immunologic activity of the isolates was determined by the enzyme allergosorbent test (EAST) and EAST inhibition assays; this activity was, in the case of Bet v I, similar to that of a preparation obtained by monoclonal antibody affinity chromatography. The allergenic potency of Bet v I and of the cross‐reactive apple allergen was determined by EAST inhibition and dose‐related histamine release. With both assay systems, the allergenic reactivity of Bet v I was considerably higher than that of the major apple allergen. Fürthermore, skin prick tests with the purified allergens and with whole allergenic extracts were performed on a group of 33 patients suffering from birch‐pollen and apple hypersensitivity, and on a control group of 10 patients. The frequency of positive prick test results in the allergic patient group ranged from 73% for the major allergen from Golden Delicious apples to 97% with Bet v I and whole birch pollen extract, respectively. In contrast to our low‐temperature extracts, commercial prick test solutions of four different manufacturers were found to be unreliable for the diagnosis of apple allergy. The skin test results again indicated the strong immunologic activity of the allergen isolates and the predominance of the major allergens in context with birch‐pollen and apple hypersensitivity. Taken together, the results support the view that the 18‐kDa major allergen represents most of the allergenicity of the the apple fruit, and that all allergenic epitopes of the apple proteins are present on Bet v I.