Histamine is an organic nitrogenous compound involved in local immune responses as well as regulating physiological function in the gut and acting as a neurotransmitter.
Histamine is involved in the inflammatory response and has a central role as a mediator of pruritus.
As part of an immune response to foreign pathogens, histamine is produced by basophils and by mast cells found in nearby connective tissues. Histamine increases the permeability of the capillaries to white blood cells and some proteins, to allow them to engage pathogens in the infected tissues.
In humans, histamine exerts its effects primarily by binding to G protein-coupled histamine receptors, designated H1 through H4.
As of 2015, histamine is believed to activate ligand-gated chloride channels in the brain and intestinal epithelium.