The Histamine reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Histamine

Histamine is a monoamine chemical involved in local immune responses. It is a hydrophilic vasoactive amine (thus the name). 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 IgE antibodies.

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.

Histamine also regulates stomach acid production.

The -amine also may function as a neurotransmitter in the central nervous system.

Histamine mediates its effects via histamine receptors, which are classified into types:

• H1 - 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 - gastric acid secretion primarily
• H3 - decreased neurotransmitter release: histamine, acetylcholine, norepinephrine, serotonin

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