histaminei

Histamine is a biogenic amine that acts as a local mediator in the immune system, as well as in the gut and nervous system. It is produced by decarboxylation of the amino acid histidine through the enzyme histidine decarboxylase, and is stored in secretory granules of mast cells and basophils, enterochromaffin-like cells in the stomach, and certain neurons. Upon release, histamine binds to four G protein–coupled receptors (H1, H2, H3, H4) on a variety of cells, producing diverse effects. Activation of H1 receptors promotes allergic inflammation, vasodilation and increased vascular permeability, bronchoconstriction, and itch. H2 receptors stimulate gastric acid secretion and modulate immune responses. H3 receptors mainly regulate neurotransmitter release in the brain and peripheral nerves. H4 receptors are implicated in immune cell chemotaxis and cytokine production.

Histamine is inactivated primarily by two enzymes: diamine oxidase (DAO), which acts extracellularly in the gut and other tissues, and histamine N-methyltransferase (HNMT), which acts intracellularly in the liver, kidney, and brain. Metabolites such as N-methylhistamine and imidazole acetaldehyde are excreted in urine.

Clinical relevance includes allergic reactions, urticaria, hay fever, and anaphylaxis, as well as histamine intolerance in individuals with reduced DAO activity. Drugs that block histamine receptors (antihistamines) or reduce histamine release, and dietary management of histamine-rich foods, are common therapeutic approaches.