Histamine

Histamine is a biogenic amine chemical involved in local immune responses as well as regulating physiological function in the gut and acting as a neurotransmitter. New evidence also indicates that histamine plays a role in chemotaxis of white blood cells.

Mechanism of Action

Histamine exerts its actions by combining with specific cellular receptors located on cells. The four histamine receptors that have been discovered are designated H1 through H4.

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• H₁ histamine receptor - found on smooth muscle, endothelium, and centeral nervous system tissue; causesvasodilation, bronchoconstriction, smooth muscle activation, and separation of endothelial cells (responsible for hives), and pain and itching due to insect stings; the 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
• H₄ histamine receptor - unknown physiological role. Found primarily in the thymus, small intestine, spleen, and colon.