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.

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histamine

an amine formed from histidine and released by the body tissues in allergic reactions, causing irritation. It also stimulates gastric secretions, dilates blood vessels, and contracts smooth muscle. Formula: C₅H₉N₃

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histamine [′his·tə‚mēn]

(biochemistry)

C₅H₉N₃An amine derivative of histadine which is widely distributed in human tissues.

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Histamine

A biologically active amine that is formed by the decarboxylation of the amino acid histidine. It is widely distributed in nature and is found in plant and animal tissues as well as in insect venoms. In humans, histamine is a mediator of inflammatory reactions, and it functions as a stimulant of hydrochloric acid secretion in the stomach.

Most tissue histamine is found stored in mast cells, where it can be released by a variety of stimuli. Once released, it can cause many effects, including constriction of bronchiolar, gastrointestinal, uterine smooth muscle, and lowering of blood pressure. If histamine is released in the skin, itching, a flare (area of redness) due to vasodilation, and a wheal due to leaking of fluid into the tissue are observed. The increase in vascular permeability that permits this leakage is due to an action on the endothelial cells of postcapillary venules.

All of these actions of histamine are mediated by the activation of histamine receptors, designated either H-1 or H-2. Antihistamine drugs exert their effects by blocking the combination of histamine with these receptors. See Antihistamine

Histamine release can be caused by tissue injury, by physical stimuli such as cold or pressure, by drugs such as heroin, and most importantly by immunologic mechanisms. Mast cells in the skin, the lung, the nasal passages, or other sites may become sensitized to antigens such as ragweed or other pollens, and then release histamine and other biologically active substances upon exposure to them. The released histamine may then cause the effects commonly associated with allergic responses. If the allergic reaction becomes generalized and severe, life-threatening anaphylactic shock may ensue. The prompt administration of epinephrine, which exerts effects opposite to those of histamine, can be life-saving in such cases. See Allergy, Antigen, Epinephrine, Hypersensitivity, Immunology