A substance that is released during allergic reactions, e.g. hay fever. Formed from the amino acid histidine, histamine can occur in various tissues but is concentrated in connective tissue. Histamine is one of the inflammatory mediators released from mast cells in response to antigen binding to IgE antibodies on the surface of the mast cell. It is a short-lived amine that causes the initial symptoms of an allergic response, such as the acute constriction of the airways in an asthmatic attack. It also causes dilation and increased permeability of small blood vessels, which results in such symptoms as localized swelling, itching, sneezing, and runny eyes and nose. The effects of histamine can be countered by the administration of antihistamine drugs.