Histamine is a compound derived from the amino acid histidine. It is found in nearly all tissues of the body, associated mainly with the mast cells. Histamine causes dilation of blood vessels and contraction of smooth muscles. It is an important mediator of inflammation and is released in large amounts after skin damage. It produces a characteristic skin reaction. Histamine is also released in anaphylactic reactions and allergic conditions, including asthma, and gives rise to some of the symptoms of these conditions.