The European Academy of Allergology and Clinical Immunology (EAACI) defines a food allergy in the following way (Johansson, 2002):

The EAACI also defines atopy:

Until now the reasons for why allergies affect only some patients and not others are not evident. It is also not known what causes an allergy to a specific allergen. A positive correlation between the rate of occurrence of major histocompatibility complex (MHC) genes (especially DR locus) and the frequency of allergies to specific allergens is being verified.