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.