Histamine

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).

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 IgE antibodies.

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

See also

• Mast cell
• Allergy