Mast cells, coloured scanning electron micrograph (SEM). Mast cells are leucocytes (white blood cells) that contain the chemical mediators histamine, serotonin and heparin. They are part of the immune system, and release their contents during a localised inflammatory immune response to invading pathogens and during an allergic reaction. Histamine and serotonin cause the symptoms of allergies. Heparin is an anticoagulant and prevents abnormal clotting. Magnification: x2667 when printed at 10 centimetres across.