Organic compound found in nearly all animal tissues, in microorganisms, and in some plants.
Its release stimulates many smooth muscles to contract, such as those in the gastrointestinal tract, uterus, and the bronchi. It causes fine blood vessels to dilate and become more permeable, causing the runny nose, watery eyes, and tissue swelling of hay fever and some other anaphylaxis ). Stinging nettles and certain insect venom s contain histamine. In humans, histamine is formed by removal of a carboxyl group from histidine . Its effects are counteracted by antihistamine s.