Lymphocyte white blood cell. Transmission electron micrograph (TEM) of a section through lymphocyte, showing the nucleus (large, round, centre), endoplasmic reticulum (ER, dark lines) and multiple mitochondria (oval). Lymphocytes are white blood cells. They are essential cells of the immune defence system that either attack foreign antigens directly or indirectly by the production of antibodies. Magnification: x28,000, when printed 10 centimetres wide.