histamine, organic compound derived in the body from the amino acid histidine by the removal of a carboxyl group (COOH). Although found in many plant and animal tissues, histamine is specifically important in human physiology because it is one of the chemicals released from certain cells (particularly mast cells) upon tissue injury or during the neutralization of foreign material (antigens) by certain types of antibodies. Released histamine tends to dilate blood capillaries, often causing the skin to appear red and feel warm, and makes the capillaries more permeable, allowing fluid to escape into the tissues. This causes edema (swelling), usually manifested as acute urticaria (rapidly appearing hives, accompanied by severe itching). This sort of reaction is common to many allergies, such as food allergies, and the symptoms can often be controlled well with antihistamines. Unfortunately, histamine is not the only substance released under these conditions, and some allergies, particularly chronic ones such as asthma, are relatively resistant to antihistamine therapy.