<Tr> <Td_colspan="2"> Infobox references </Td> </Tr> <Tr> <Td> </Td> <Td> </Td> </Tr> <P> 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 for the brain, spinal cord, and uterus . Histamine is involved in the inflammatory response and has a central role as a mediator of itching . 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 . </P> <P> Histamine base, obtained as a mineral oil mull, melts at 83--84 ° C. Hydrochloride and phosphorus salts form white hygroscopic crystals and are easily dissolved in water or ethanol, but not in ether . In aqueous solution, histamine exists in two tautomeric forms: N - H - histamine and N - H - histamine . The imidazole ring has two nitrogens . The nitrogen farthest away from the side chain is the' tele' nitrogen and is denoted by a lowercase tau sign . The nitrogen closest to the side chain is the' pros' nitrogen and is denoted by the pi sign . The position of the nitrogen with the hydrogen on it determines how the tautomer is named . If the nitrogen with the hydrogen is in the tele position, then histamine is in the tele - tautomer form . The tele - tautomer is preferred in solution . </P>

Where does histamine come from in the body