An allergy is an inappropriate response by the body's immune system to a substance that is not normally harmful. The immune system is a highly complex defense mech;:Inism that helps us to fight infections. It does this by identifying foreign invaders and mobilizing the body's white blood cells to fight them. In some people, the immune system wrongly identifies a nontoxic substance as an invader, and the white blood cells overreact, creating more damage to the body than the invader. Thus, the anergic reponse becomes a disease in itself.