Individuals who react to specific food allergens, inhalants or substances can develop an allergy to others. Such reactions may be to different foods containing the same allergen, or to an allergen with a very similar protein structure. Reactions can be either light or heavy and are known as allergic cross-reactivity.

This means that someone may suffer an allergic reaction even when avoiding the foods they know they are allergic to. If someone is allergic to peanuts, for example, they might react to soya, peas, lentils or beans – food items in the same biological family (legume).

Allergic cross-reactions can also happen between certain fruit or vegetables and latex (known as latex-food syndrome), or the pollens that cause hay fever.
If a person has a measurable reaction to a certain food with clinical diagnosis, avoiding similar foods that could trigger this reaction may be helpful.

The most well-documented cross-reactivity occurs between apple and birch pollen; however, individuals who are allergic to apple are not necessarily allergic to birch pollen. Cross-reactivity should not be assumed, and important foods should not be eliminated from the diet without appropriate testing and clinical diagnosis.