Histamine
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Histamine is a monoamine chemical involved in local immune responses.
Chemical properties
Chemically, histamine is 2-(4-imidazolyl)ethylamine and has the formula C5H9N3. Histamine is synthesized by the decarboxylation of the amino acid histidine, a reaction catalyzed by the enzyme L-histidine decarboxylase. It is a hydrophilic vasoactive amine (thus the name). Some forms of food poisoning are due to conversion of histidine into histamine in spoiled foodstuffs, such as fish.
Physiology
Release of histamine is associated with allergic reactions, as it causes both an inflammatory response and a contraction of smooth muscle tissue. Histamine is released by mast cells primarily when they degranulate due to binding of allergens to IgE antibodies on the mast cell.
Histamine also regulates gastric acid production. It is released by ECL cells in the stomach to stimulate parietal cells to release HCl.
It may also function as a neurotransmitter in the central nervous system.
Histamine mediates its effects via histamine receptors, which are classified into types:
- H1 histamine receptor - vasodilation; bronchoconstriction; smooth muscle activation; separation of endothelial cells, responsible for hives; pain and itching due to insect stings; primary receptors involved in allergic rhinitis symptoms and motion sickness
- H2 histamine receptor - located on parietal cells, which primarily regulate gastric acid secretion
- H3 histamine receptor - decreased neurotransmitter release: histamine, acetylcholine, norepinephrine, serotonin