Mast cells. Light micrograph of several mast cells (pink rounded objects). These leucocytes (white blood cells) contain the chemical mediators histamine, serotonin and heparin. Part of the immune system, they release their contents during a localized inflammatory immune response to invading pathogens and during an allergic reaction. Heparin is an anticoagulant which prevents abnormal clotting. Histamine and serotonin cause the symptoms of allergies.