Histamine is a protein involved in local immune responses. It is synthesized from the amino acid histidine and is a 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.

Histamine also regulates stomach acid production.

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