Food allergy is an immune system reaction that occurs soon after eating a certain food. Even a tiny amount of the reaction-causing food can trigger signs and symptoms such as digestive problems, hives or swollen airways. This happens because the immune system overreacts to the proteins in that food. While a person can be allergic to any food, the nine most common allergens are cow’s milk, egg, peanuts, wheat, soy, fish, shellfish, sesame, and tree nuts.