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. Two types of white blood cell (purple) can also be seen. They are a neutrophil (upper centre right) and a lymphocyte (upper centre left). White blood cells are part of the body's immune system. Neutrophils are the most abundant white blood cell. They engulf and digest invading bacteria. 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.