The primary cells of the lymphatic system are lymphocytes. There are three types of lymphocytes: T cells, B cells, and NK cells. T cells account for approximately 80 percent of the circulating lymphocytes. They are thymus-dependent and are the primary cells that provide cellular immunity. B cells, which are derived from the bone marrow, account for 10 to 15 percent of the circulating lymphocytes. They are responsible for antibody-mediated immunity. NK (natural killer) cells account for the remaining 5 to 10 percent of the circulating lymphocytes. They attack foreign cells, normal cells infected with viruses, and cancer cells that appear in normal tissues.