"Life-saving cancer treatments can occasionally cause distressing or uncomfortable side effects that patients can find difficult to cope with. Extracorporeal photopheresis (ECP) is a relatively new treatment that’s helping to improve the quality of life for dozens of cancer patients every year ... ECP is most commonly used to treat two conditions - Cutaneous T cell Lymphoma (CTCL) and Chronic Graft versus Host Disease (GvHD)."
A good match is needed between host and donor to reduce the risk of graft-versus-host disease (GVHD). As more transplants were performed for CML, a curious phenomenon was seen. If people survived GVHD, they typically did better and were less likely to suffer a relapse of their #CML. Read more about the the graft vs. leukemia effect: http://cml-iq.com/transplantation-the-graft-vs-leukemia-effect/