Biochemistry. Bioactive amine formed by the decarboxylation of histidine. Histamin is found in all human tissues. It is a potent mediator (hormone-like substance) affecting receptors in smooth muscles and in secretory systems. Its release is caused by allergig reaction, inflammation and burns. Histamin induces the dilation of capillaries, the contraction of most smooth muscle tissue (including arterial and bronchial walls), increased gastric secretion, acceleration of heart rate and nervous sensations like pain and itching.