andlt;strongandgt;Mast cell, andlt;/strongandgt; coloured transmission electronmicrograph (TEM). Mast cells are a type of whiteblood cell found in connective tissue. The largeoval (orange) is the cellandamp;apos;s nucleus, whichcontains its genetic information. Within thecellandamp;apos;s cytoplasm (green) are granules (red)containing chemical mediators, including histamineand heparin. When the mast cell is activated, either by an allergic reaction or in response toinjury or inflammation, these granules arereleased into the tissues. Histamine isresponsible for the symptoms of an allergicreaction. It causes pain and itching, dilatescapillaries and makes them more permeable, leadingto red skin and swelling. Heparin is ananticoagulant, it prevents blood from clotting.
