Blood cells. Coloured scanning electron micrograph (SEM) of red blood cells (erythrocytes, red) and a white blood cell (lymphocyte, blue). These are the two main cell components of blood. Erythrocytes are carriers of oxygen and carbon dioxide. Their biconcave surface provides maximum surface area for the exchange of gases. Lymphocytes are involved in the immune system's defence mechanisms, lymph system, and involved in antibody production. Magnification: x6500 when printed 10cm wide.