Color enhanced scanning electron micrograph (SEM) of human blood cells. Three components of blood are seen: erythrocytes (red), lymphocytes (white), and platelets (blue). Erythrocytes or red blood cells are carriers of oxygen and carbon dioxide. Their biconcave shape provides maximum surface area for the exchange of gases. Lymphocytes or white blood cells play a central role in immune defence mechanisms, fighting pathogens in the blood and lymph system. While platelets are not true cells, but membrane bound cell fragments of the bone marrow which help the blood clot. Platelets may take on many shapes and sizes.