Can any protein become an allergen?

Allergens of plant and animal foods and pollen belong to a highly restricted number of protein families. The AllFam Database (http://www.meduniwien.ac.at/allergens/allfam/) provides regularly updated lists of protein families that contain allergens. At present, 2% of the 9318 protein families defined by the Pfam Database (http://pfam.sanger.ac.uk/) contain allergens. Related protein families can be grouped into superfamilies placing allergenic proteins in an evolutionary context. With the exception of the prolamin superfamily, allergenic plant proteins are found in few member families of their respective superfamilies. This might indicate that allergenicity emerged rather infrequently in a very limited number of protein families. Moreover, most members of a given protein family seem to be non-allergenic. In contrast to plant allergens, the allergenicity of animal food allergens seems to be dependent on the degree of identity to a human homologue. The closer a potential animal allergen is to a human protein, the less likely it is to act as allergen.