False-colour transmission electron micrograph of a human mast cell in tissue. Mast cells are found in connective tissue and form part of the body's immune system. Within the cell is a large circular nucleus coloured red, containing strands of chromatin coloured blue. In the surrounding cytoplasm coloured yellow are numerous small red granules. 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 reactions. Magnification: x1740 at 6x4.5cm size.