Coloured scanning electron micrograph (SEM) of mast cells from the human intestine that have been activated by immunoglobulin E. Mast cells are a type of white blood cell that are found in connective tissue. Within the cell's 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: x4,000 when printed at 15 centimetres wide.