Blood clot, coloured scanning electron micrograph (SEM). Red blood cells (erythrocytes) and white blood cells (leucocytes, green) trapped within the fibrin mesh (yellow) of a fresh blood clot. Clots are formed in response to cardiovascular disease or injuries to blood vessels. White blood cells commonly occur in blood clots, fulfilling their immune system role in defending the body from invading bacteria, viruses and foreign substances. Magnification: x2000 when printed 10 centimetres wide.