Histamine, which is formed by decarboxylation of the amino acid histidine, is a major component of mast-cell granules, accounting for about 10% of granule weight. Because it is stored—preformed—in the granules, its biological effects are observed within minutes of mast-cell activation. Once released from mast cells, histamine initially binds to specific receptors on various target cells. Three types of histamine receptors—designated Hj, H2, and H3—have been identified; these receptors have different tissue distributions and mediate different effects when they bind histamine.
Most of the biologic effects of histamine in allergic reactions are mediated by the binding of histamine to H receptors. This binding induces contraction of intestinal and bronchial smooth muscles, increased permeability of venules, and increased mucus secretion by goblet cells. Interaction of histamine with H2 receptors increases vasopermeability and dilation and stimulates exocrine glands. Binding of histamine to H2 receptors on mast cells and basophils suppresses degranulation; thus, histamine exerts negative feedback on the release of mediators.
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If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.