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, from left to right; a monocyte, a neutrophil and a lymphocyte. White blood cells are part of the body's immune system. Monocytes are the largest white blood cell. They engulf and digest invading bacteria and cell debris. 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. Magnification: x1,100 when printed at 10 centimetres wide.