Polarised light micrograph of crystals of the chemical histamine. Histamine is derived from the amino acid histidine, and is mainly associated with the mast cells found in connective tissue. It is one of the chemicals involved in inflammation, causing dilation and increased permeability of small blood vessels. It is released in large quantities after damage to the skin (eg in response to animal or insect venom), producing a characteristic red wheal. Histamine is also released in and gives rise to the symptoms of a number of allergic conditions, including asthma. Magnification unknown.