stored in mast cells and released by antigen. (see hypersensitivity). Responsible for the early symptoms of anaphylaxis. Also present in some venoms. Amine formed from histidine that stimulates gastric secretions and dilates blood vessels; released by the humane immune system during allergic reactions.A chemical that is produced particularly by leucocytes that results in blood vessels becoming more permeable and in turn lose fluid to the tissues. This can result in localised swelling, and is a direct response to the presence of an [[antigen.