Mast cell, coloured transmission electron micrograph (TEM). Mast cells are a type of white blood cell found in connective tissue. The large oval (orange) is the cell's nucleus, which contains its genetic information. Within the cell's cytoplasm (green) are granules (red) 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.