On the second day of radiation (day -3), the subject will receive CAMPATH-1H as a daily 4-hour IV (intravenous, by vein). The subject will receive this infusion once a day for a total of three days. CAMPATH 1H is a special type of protein called an antibody, that works against certain types of blood cells. CAMPATH 1H is important because it stays active in the body for a long time after infusion, which means it may work longer at preventing GVHD symptoms.