A food allergy is when the body mistakenly believes that food that has been consumed is harmful, and the immune system responds to this perceived threat. After the first exposure to the food, the body creates antibodies that are meant to fight the particular food. Any time that the food is then eaten, the immune system fights it, releasing chemicals, such as histamine, as a protective reaction. This response by the immune system causes the allergic symptoms, which can affect the skin, breathing, the gastrointestinal tract, and the heart and cardiovascular system.

The exact sources of allergies are not fully understood. Genes, passed on from parent to child, may make some more likely to have food allergies. Multiple exposures to the allergen will then cause the allergy to appear in those with this genetic predisposition. Some believe that parents can pass on specific allergies, though only in rare cases. Some studies have suggested that exclusive breastfeeding, while the mother avoids major food allergens, can prevent or reduce the risk that the infant will suffer from the allergy. Smoking, on the other hand, seems to increase the likelihood of allergies in the baby.