Histamine is an amine with the chemical compositions C5H9N3. In humans, it is found in two general sites.  It is throughout the body in mast cells and in basophils circulating in the blood.  The second site is in cells located within the central nervous system (CNS).

The histamine found in mast cells and basophils serves a function in the early immune response. It is released as part of an allergic reaction.

The histamine found in the CNS serves as a neurotransmitter.  It is a promoter of wakefulness. Cells in the Posterior hypothalamus – tuberomammallian nucleus (TMN) – release histamine and the histamine stimulates wakefulness

The chemical structure of histamine.

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