A food allergy is an allergic reaction of the immune system to an otherwise harmless vegetable or animal protein in food (allergen).
Often, very small amounts of an allergen are sufficient to trigger an allergic reaction. With severe allergy sufferers, the amounts may be in the microgramm range.
Eight foods cause most of all food allergies: Cow's milk, peanuts, eggs, tree nuts, wheat, fish, soy, seafood.
Effectively, any vegetable or animal protein can cause allergic reactions; the above are the most common causes, though.