Substance that plays a major role in many allergic reactions. Histamine dilates blood vessels and makes the vessel walls abnormally permeable.
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A vasodepressor amine derived from histidine by histidine decarboxylase and present in ergot and in animal tissues. It is a powerful stimulant of gastric secretion, a constrictor of bronchial smooth muscle, and a vasodilator (capillaries and arterioles) that causes a fall in blood pressure. H., or a substance indistinguishable in action from it, is liberated in the skin as a result of injury. When injected intradermally in high dilution, it causes the triple response.
- h. phosphate used in the treatment of certain allergies, cephalalgia, and acute multiple sclerosis with varying results; also used to test gastric secretory function, in the diagnosis of pheochromocytoma and in the treatment of Ménière disease; also available as h. acid phosphate.

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his·ta·mine 'his-tə-.mēn, -mən n a compound C5H9N3 esp. of mammalian tissues that causes dilatation of capillaries, contraction of smooth muscle, and stimulation of gastric acid secretion, that is released during allergic reactions, and that is formed by decarboxylation of histidine
his·ta·min·ic .his-tə-'min-ik adj

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a compound derived from the amino acid histidine. It is found in nearly all tissues of the body, associated mainly with the mast cell. Histamine has pronounced pharmacological activity, causing dilation of blood vessels and contraction of smooth muscle (for example, in the lungs). It is an important mediator of inflammation and is released in large amounts after skin damage (such as that due to animal venoms and toxins), producing a characteristic skin reaction (consisting of flushing, a flare, and a weal). Histamine is also released in anaphylactic reactions and allergic conditions, including asthma, and gives rise to some of the symptoms of these conditions. See also anaphylaxis, antihistamine.

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his·ta·mine (hisґtə-mēn) chemical name: 1H-imidazole-4-ethanamine. A decarboxylation product of histidine, C5H9N3, found in all body tissues, particularly in the mast cells and their related blood basophils, the highest concentration being in the lungs. It is also present in ergot and other plants and may be synthesized outside the body from histidine or citric acid. It has several functions, including (1) dilation of capillaries, which increases capillary permeability and results in a drop of blood pressure, (2) contraction of most smooth muscle tissue, including bronchial smooth muscle of the lung, (3) induction of increased gastric secretion, and (4) acceleration of the heart rate. It is also responsible for the triple response, and is implicated as a mediator of immediate hypersensitivity. Cellular receptors of histamine include three types: the H1 receptors mediate the contraction of smooth muscle and the effects on capillaries; the H2 receptors mediate the acceleration of heart rate and the promotion of gastric acid secretion. Both H1 and H2 receptors mediate the contraction of vascular smooth muscle. H3 receptors occur in a number of systems including the central nervous system and peripheral nerves, and are believed to play a role in regulation of the release of histamine and other neurotransmitters from neurons.

Medical dictionary. 2011.

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