histamine

histamine (hĭs`təmēn'), 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.

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histamine

Organic compound found in nearly all animal tissues, in microorganisms, and in some plants. Its release stimulates many smooth muscles to contract, such as those in the gastrointestinal tract, uterus, and the bronchi. It causes fine blood vessels to dilate and become more permeable, causing the runny nose, watery eyes, and tissue swelling of hay fever and some other allergies. Histamine appears to have a physiological role in the body's defenses against a hostile environment, since it may be released when the body is subjected to trauma, infection, or some drugs. Under extreme circumstances, the effects of histamine lead to exaggerated responses with distressing results, as may occur in some allergic conditions (see anaphylaxis). Stinging nettles and certain insect venoms contain histamine. In humans, histamine is formed by removal of a carboxyl group from histidine. Its effects are counteracted by antihistamines.