A food allergy occurs when the immune system responds to a harmless food as if it were a threat. The first time a person with food allergy is exposed to the food, no symptoms occur. But the body has been now been primed, and when the person eats the food again, an allergic response occurs.
An allergic reaction to food usually takes place within a few minutes to several hours after exposure to the allergen. The process of eating and digesting food and the location of immune cells involved in the allergic reaction process both affect the timing and location of the reaction.