Coloured transmission electron micrograph (TEM) of a section through a mast cell, with vacuoles (bright yellow) containing histamine or heparin. A mast cell is a leucocyte (white blood cell) that contains the chemical mediators histamine, serotonin and heparin. It is part of the immune system, and releases its 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 which prevents abnormal clotting. Magnification: x7700 when printed at 10 centimetres across.