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Background. Hazelnut major allergens up to now identified are a 18 kd protein homologous to Bet v 1 and a 14-kd allergen homologous to Bet v 2. No studies reported hazelnut allergens as recognized by patients with positive double-blind placebo controlled food challenges (DBPCFCs) or allergic to hazelnut but not to birch. Objective. In this study we characterized the hazelnut allergens by studying the IgE reactivity of 65 patients with positive DBPCFC and of 7 patients with severe anaphylaxis to hazelnut. Methods. Hazelnut allergens were identified by SDS-PAGE and IgE immunoblotting. Further characterization was made by: a) amino-acid sequencing; b) evaluation, with EAST inhibition, of IgE binding properties of raw and roasted hazelnut, and c) assessment of cross- reactivity with different allergens by immunoblotting inhibition. Results. All the sera from the DBPCFC positive patients recognized a 18 and a 47 kd allergen; other major allergens were at a m.w. of 32 and 35 kd. The binding to the 18 kd band was inhibited by birch extract, demonstrating its homology with the birch major allergen, and was abolished in roasted hazelnut. The 47 kd allergen is a sucrose-binding protein; the 35 kd allergen is a legumin; the 32 kd allergen is a 2S albumin. The patients with severe anaphylactic reactions to hazelnut showed a specific IgE reactivity to a 9 kd allergen, totally inhibited by purified peach LTP, which was heat-stable. Conclusions The major allergen of hazelnut is a 18 kd protein, homologous to Bet v 1; the 9 kd allergen is likely a lipid transfer protein; other major allergens have a m.w. of 47, 32 and 35 kd.