A high number of birch pollen-allergic individuals develop hypersensitivity reactions to certain foods, e.g. apples. This food allergy is due to immunological cross-reactivity. Birch pollen-related foods contain proteins, e.g. Mal d 1 in apple, that are structurally related with the major birch pollen allergen, Bet v 1. Hence IgE antibodies and T lymphocytes specific for Bet v 1 recognize these food proteins which results in activation of the immune system and, consequently, in clinical symptoms.