Color enhanced scanning electron micrograph (SEM) of a white blood cell. There are three main types of white cells - lymphocytes, leukocytes and monocytes. Lymphocytes originate in the bone marrow & mature there or in the thymus - hence the names B-lymphocytes and T- lymphocytes. B-cells produce antibodies, chemicals that disable invading microorganisms. Some T-cells attack other cells infected with viruses directly.