False-colour transmission electron micrograph of a mast cell (central green body) from a human tumour. Mast cells are found in connective tissue and form part of the body's immune system. The cell contains a large oval nucleus (red and dark green), surrounded by three large, fat globules (fuzzy red) and a number of small, round, granules (red). The granules contain the chemicals heparin, histamine and serotonin. These are released in response to foreign proteins on the surfaces of invading bacteria or viruses and are responsible for setting off inflammation and allergic responses. Magnification: x2835 at 6x4.5cm size.