Mast cell. Coloured transmission electron micrograph (TEM) of a section through a mast cell. This leucocyte (white blood cell) contains the chemical mediators histamine, serotonin and heparin, held in granules (red) in its cytoplasm. Mast cells are part of the immune system; they release their contents during a localized inflammatory immune response to invading pathogens, and during an allergic reaction. Histamine and serotonin cause the symptoms of allergies. Heparin is an anticoagulant that may prevent the formation of blood clots between the body's cells. Magnification unknown.