A vasodepressor amine derived from histidine by histidine decarboxylase and present in ergot and in animal tissues. It is a powerful stimulant of gastric secretion, a constrictor of bronchial smooth muscle, and a vasodilator (capillaries and arterioles) that causes a fall in blood pressure. Histamine, or a substance indistinguishable in action from it, is liberated in the skin as a result of injury. When injected intradermally in high dilution, it causes the triple response.
