In mammalians, histamine signals numerous physiological functions. In humans, histamine is released by immune mast and basophil cells, but also by gut bacteria. Histamine (2-(1H-imidazol-5-yl)-ethanamine) is biosynthesized by decarboxylation of histidine and metabolized by imidazole-N-methyltransferase methylation and/or diaminooxidase deamination. Low µM concentrations of histamine are normally present in lung, skin, heart, gut and nervous system .