Mast cell. Transmission electron micrograph (TEM) of section through a mast cell. Mast cells are specialised white blood cells (leukocytes) that settle in connective tissue and usually do not circulate in blood. The large dense granules (dark circles) in the cell cytoplasm contain many biologically important molecules including histamine, which causes tissue swelling in inflammatory reactions, and heparin, which acts as an anticoagulant. The typical symptoms of an allergic reaction are due in large part to the release of mast cell granules. Magnification: x19,000, when printed 10 centimetres wide.