Food allergens.

A food allergen may be defined as a substance that reacts with IgE antibodies, induces allergic sensitisation or induces allergic reactions. Some allergens only induce allergic sensitisation but do not provoke symptoms, while others bind IgE but do not induce mast cell degranulation. There is no common structure that can predict whether a given antigen may be a strong food allergen. A complete food allergen, e.g. fish parvalbumin, is capable of stimulating the immune system to produce IgE antibodies, and degranulate mast cells upon subsequent contact. The reason(s) for why some patients with IgE to ovalbumin tolerate eggs, and why some react on one occasion but not on another, are mostly unclear, but may be related to changes in gut permeability induced by other food substances or by gastro-intestinal inflammation prior to the allergen contact. IgE antibodies to fruit or vegetables often show cross-reactivity, due to carbohydrate structures. These cross-reactive glycans have been designated cross-reactive carbohydrate determinants (CCD). Anti-CCD antibodies are highly cross-reactive. The antibodies do not have clinical significant because CCD-containing foods are usually well-tolerated by patients with IgE antibodies to CCD. These IgE antibodies may cause confusion in relation to allergy diagnosis.