Blood cells. Coloured scanning electron micrograph (SEM) of a human red blood cell (erythrocyte, red) and a white blood cell (leucoocyte, 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. The lymphocyte is involved in the immune system's defence mechanisms, protecting the body against the invasion of foreign pathogens in the blood and lymph system, and involved in antibody production. Magnification: x15,000 when printed 10cm wide.