Histamine is a neurotransmitter that regulates inflammatory physiological functions in the gut. Stimulates gastric secretion, bronchial smooth muscle constriction, and vasodilation. Increases the permeability of capillaries for white blood cells and other proteins to engage foreign pathogens. Potent agonist of the G protein coupled histamine receptors H1-4. Released by mast cells and basophils, and is derived by enzymatic carboxylation of the amino acid histidine.