Activated mast cell, coloured scanning electron micrograph (SEM). Mast cells are a type of white blood cell that are found in connective tissue. Within the cells cytoplasm are granules containing chemical mediators, including histamine and heparin. When the mast cell is activated, either by an allergic reaction or in response to injury or inflammation, these granules are released into the tissues. Histamine is responsible for the symptoms of an allergic reaction. It causes pain and itching, dilates capillaries and makes them more permeable, leading to red skin and swelling. Heparin is an anticoagulant, it prevents blood from clotting. Magnification: x1750 when printed 10 centimetres wide.