False-color scanning electron micrograph (SEM) of human red blood cells (erythrocytes) and a single white blood cell (leucocyte). These are the two main cell components of blood. At top, erythrocytes are carriers of oxygen and carbon dioxide. Their biconcave surface provides maximum surface area for the exchange of gases. A leucocyte (white, at left) is involved in immunological defence mechanisms protecting the body against the invasion of foreign pathogens in the blood and lymph system, and involved in antibody production. Leucocytes include the class of lymphocyte cells (T-cells & B-cells). Magnification: x650 at 6x7cm x325 at 35mm size.