As you may have known already, food allergy is an unusual reaction to food that is elicited by an exposure to certain food types which then creates a response from the immune system. The reaction that the immune system eventually generates will then cause discomfiting symptoms.
Some people are not aware that they have food allergy. It can be difficult to diagnose especially if allergic reactions are not apparent before. Symptoms of food allergy only appear if you exceed the tolerable amount of foods you are allergic to.