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. Three types of white blood cell (purple) can also be seen. They are, an eosinophil (bottom left), a lymphocyte (lower right) and a monocyte (upper centre). White blood cells are part of the body's immune system. Eosinophils are involved in the body's allergic response and also help to defend the body from invading parasites. Lymphocytes are involved in the production of antibodies and attacking virus-infected and tumour cells. Monocytes are the largest white blood cell. They engulf and digest invading bacteria and cell debris. Also seen are platelets (small purple particles), which are cell fragments that play an essential role blood clotting. Magnification: x1000 when printed at 15cm wide.