Blood smear. Light micrograph showing human blood cells. Erythrocytes (red blood cells) are pink. They make up the majority of blood cells and carry oxygen to body tissues and carbon dioxide to the lungs. At centre is a lymphocyte white blood cell (purple). White blood cells are part of the body's immune system. Lymphocytes are involved in the production of antibodies and attacking virus-infected and tumour cells. Also seen are platelets (small purple particles), which are cell fragments that play an essential role blood clotting. Magnification: x1,200 when printed at 15cm wide.