Histamine is a biogenic amine derived from the decarboxylation of the amino acid L-histidine. It plays critical roles in the immune system as a mediator of allergic responses, in gastric acid secretion, and as a neurotransmitter in the central nervous system. Released from mast cells and basophils during inflammation or allergic reactions, it binds to H1-H4 receptors causing vasodilation, increased vascular permeability, smooth muscle contraction, and itching. Chemically, it consists of an imidazole ring attached to an ethylamine chain. It is endogenous in humans and animals but can trigger scombroid poisoning when ingested in high amounts from spoiled fish. It is highly polar and water-soluble.