ALEMTUZUMAB IS A HUMANISED DERIVATIVE OF CAMPATH-1G, A RAT MONOCLONAL ANTIBODY TO THE CD52 ANTIGEN FOUND ON LYMPHOCYTES. IT BINDS TO CD52, AN ANTIGEN PRESENT ON THE SURFACE OF B AND T LYMPHOCYTES, A MAJORITY OF MONOCYTES, MACROPHAGES, NK CELLS, AND A SUBPOPULATION OF GRANULOCYTES. A PROPORTION OF BONE MARROW CELLS, INCLUDING SOME CD34+ CELLS, EXPRESS VARIABLE LEVELS OF CD52. THE PROPOSED MECHANISM OF ACTION IS ANTIBODY-DEPENDENT CELLULAR-MEDIATED LYSIS FOLLOWING CELL SURFACE BINDING OF CAMPATH TO THE LEUKEMIC CELLS.