Most of the side effects experienced by people who receive stem cell transplantations for hematologic cancers are caused by the high-dose chemotherapy and radiotherapy administered prior to the transplantation. The most common and serious are due to the destruction of essentially all bone marrow cells, leading to dramatically reduced numbers of red blood cells, white blood cells, and platelets. Infections and bleeding are particularly likely. The most feared side effect of the transplantation itself is graft-versus-host disease (GVHD), a disorder in which white blood cells in the stem cell preparation attack and destroy cells of the person receiving the transplant. GVHD most often causes destruction of cells in the skin, liver, and intestines, and it can be fatal. The risk of GVHD can be reduced by receiving drugs to suppress the function of the white blood cells or by treating the stem cells to remove the accompanying white blood cells. It only occurs with allogeneic transplantations, in which the stem cells are genetically different from the person receiving them: that is, they are obtained from another person, who is not an identical twin. Graft-versus-tumor is a related, but beneficial, effect that may also occur. In this situation, the white cells in the stem cell preparation attack and destroy any remaining cancer cells.