Histamine is a biologically active amine compound formed in the body by removal of an acid group from histidine. It is most often associated with Type 1 IgE hypersensitive reactions.

When released from basophils and tissue mast cells, the biological effects of histamine include increased vascular permeability of small venules; contraction of bronchial and other smooth muscle; increased gastric, nasal, and lacrimal secretions. A function not normally associated with histamine is the role of neurotransmitter in the brain. Elevated histamine levels may also be related to certain forms of headache and schizophrenia. A list of the characteristics of patients with low or high blood histamine levels are shown in Table 1 (p.237).