A cross-reactivity allergy is present when the antibodies against a specific allergen
are also capable of identifying other allergens from other allergen sources and may thus induce an allergic reaction to those allergens as well. Cross-reactions are frequently seen between certain pollen types and foods. This is called "oral allergy syndrome". For example, some people reacting allergic to birch pollen may not tolerate apples as well. Antibodies targeting a birch pollen allergen detect a similar protein in the apple. The result is that people suffering from birch pollen allergy may react to the consumption of apples with allergic symptoms such as swelling, redness and itching of the oral mucosa, although their original immune response was not targeting the apple antigen but the main allergen in birch pollen. The following list contains a selection of relevant cross-reactive allergens, but does not lay any claim to completeness.