Allogeneic stem cell transplantation is the process of transferring stem cells from a donor (sibling, relative, or unrelated donor) other than the patient themselves. Unlike autologous transplants, allogeneic transplants create a new immune system response to fight cancer. In this process, immune system cells are transplanted along with the donor's stem cells. However, if the donor's lymphocytes perceive some cells in the recipient's body as foreign, they may initiate a reaction against them. This can lead to "graft-versus-host" disease (GVHD), which can cause severe damage to the recipient's organs. To mitigate this risk, immunosuppressive drugs are administered to the patient after transplantation to try to prevent the donor's lymphocytes from reacting. To minimize the risk of GVHD, the highest possible degree of tissue compatibility between the patient and the donor is aimed for.