1. An amine (chemical compound containing nitrogen, derived from ammonia) formed from the amino acid histidine (one of the 20 building blocks of protein) by decarboxylation (removal of the carboxyl [univalent radical characteristic of all organic acids] group from a molecule) and produced mainly by the mast cells in connective tissue as a response to injury or allergic reaction.

It causes contraction of smooth muscle, stimulates gastric secretion of hydrochloric acid and pepsin, and dilated blood vessels, which lowers blood pressure and produces inflammation, itching, or allergic symptoms; such as, sneezing.

2. A chemical in the body tissues which is produced by the breakdown of histidine.

It is released in allergic reactions and causes widening of capillaries, decreased blood pressure, increased release of gastric juice, fluid leakage forming itchy skin and hives, and tightening of smooth muscles of the bronchial tube and uterus.

3. An amine released by immune cells that produces allergic reactions.

Any organic derivative of ammonia formed by the replacement of hydrogen with one or more alkyl groups.

A drug that blocks the stimulation of cells by histamine.

This type of agent acts by interfering with the action of histamine rather than by preventing its secretion.

Water-soluble colorless crystals, sometimes called histamine acid phosphate or histamine diphosphate.

It is used most frequently as a diagnostic agent in determining the acid-secreting power of the stomach.