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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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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