Physiologic Definition: An allergy is a peculiar response on the part of the body, an attempt to protect that leads to an injury. The way it works is this: A foreign substance, an antigen, finds its way into the body, is picked up by special cells called macrophages, which can only partially digest the antigens. When the macrophages have worked on the antigen as much as they can, it is regurgitated into the bloodstream where it is picked up by a second group of special cells called lymphocytes. These cells are designed to prepare antibodies against antigens. Antibodies are capable of neutralizing antigens. When the antigen and antibody unite, a third compound is formed which in some instances turns out to be the villain in allergies. Because the third compound injures the body, the cells in the skin, digestive tract, or nasal passages may release chemicals of injury which act on various target tissues. As a result of injury to the target tissues, a sort of reflexive action occurs—such as spasms, secretions of mucus, swelling of tissue, and inflammation.