BackgroundIgE antibodies specific for the major birch pollen allergen frequently cross-react with Bet v 1 homologous food proteins, for example Cor a 1 in hazelnut and Mal d 1 in apple. Specific immunotherapy with birch pollen (BP-SIT) induces IgG4 antibodies that inhibit IgE binding to Bet v 1. However, information on cross-reactivity of BP-SIT-induced Bet v 1-specific IgG4 antibodies with food allergens is limited. In this study, we investigated the kinetics of production, cross-reactivity, and IgE-blocking activity of Bet v 1-specific IgG4 antibodies emerging during conventional BP-SIT and whether IgG4-epitopes overlapped with IgE epitopes. MethodsIgE and IgG4 levels specific for Bet v 1, Mal d 1, and Cor a 1 were determined in 42 birch pollen-allergic patients before and during BP-SIT. Inhibition of IgE binding was studied by IgE-facilitated antigen-binding assays and basophil activation tests. Furthermore, inhibition of IgE-mediated activation of food allergen-reactive Bet v 1-specific T-cell lines was assessed. Competitive immunoscreening of phage-displayed peptides was applied to select mimotopes recognized by IgE and IgG4 antibodies, respectively. The resulting mimotopes were mapped on the surface of the 3D structure of the allergens using a computer-based algorithm. ResultsBP-SIT significantly increased Bet v 1- and food allergen-reactive IgG4 antibodies. In parallel, allergen-specific IgE levels decreased significantly. Sera containing food allergen-reactive IgG4 antibodies inhibited IgE binding, basophil activation, and IgE-mediated food allergen-induced T-cell proliferation. Predicted IgE and IgG4 epitopes on all allergens showed high overlap. ConclusionOur results indicate that BP-SIT may induce Bet v 1-specific IgG4 antibodies that cross-react with related food allergens and inhibit IgE binding by epitope competition.