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Histamine, a decarboxylation product of histidine, is a biochemical substance (C5H9N3) involved in many physiological processes. Histamine functions as a neurotransmitter in the central nervous system, affects smooth muscle and gastric acid secretion, and has an important role in the immune system.

Histamine is released in almost all tissues of the body when the tissue becomes damaged or inflamed. Histamine is also released when allergic reactions occur. Most of the histamine released comes from mast cells in the damaged tissues and basophil granulocytes in the blood.

Histamine has a great vasodilator effect on the arterioles, which means the blood vessels widen. It also greatly increases capillary porosity, which allows fluid and plasma proteins to leak into the tissues. A negative side-effect of these characteristics is that edema can occur in many pathological conditions as a direct result of the increased blood flow and porosity. These characteristics are most prominent when allergic reactions occur.

There are three known classes of histamine receptor, and histamine antagonists for the first two types are in wide clinical use. histamine1 blockers are commonly called "antihistamines", with applications for allergy and as an antiemetic. Histamine2 are most commonly used for gastric distress, but also can help relax smooth muscle, as in bronchospasm. Histamine3 antagonists are in research for a variety of central nervous system conditions.