CD52 is a protein present on the surface of mature lymphocytes. It also is found in monocytes and dendritic cells. It is a glycosylphosphatidylinositol (GPI)--anchored antigen expressed on the all lymphocytes as well as within the male genital tract.The CAMPATH-1 family of monoclonal antibodies recognize an antigen expressed on human lymphocytes and monocytes. The antigen is an unusually good target for complement-mediated attack and, for this reason, the IgM antibody, CAMPATH-1M, has been widely used for removal of T lymphocytes from donor bone marrow to prevent graft-vs-host disease. Because the target antigen is expressed in most cases of lymphoid malignancy, serotherapy of lymphoma and leukemia with CAMPATH-1 antibodies has also been attempted.