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The donor is an identical twin of the patient. This is the simplest source of stem cells. Syngeneic transplants are the least complicated transplants because there is no risk of rejection, graft-versus-host disease (GVHD), or tumor in the marrow. Blood cell recovery and return of immune system functioning is prompt. The only disadvantage of syngeneic transplants is the lack of the graft versus leukemia (GVL) effect of allogeneic transplants that helps reduce tumor relapse.
An allogeneic donor is one other than the patient. Allogeneic transplants have the lowest risk of tumor relapse because of the GVL effect. However, GVHD (where the new transplanted immune system sees the patient as foreign), graft failure, and immune deficiency are potential problems. Allogeneic donors are chosen based on a blood test for human leukocyte antigens (HLA). HLA antigens are part of the biological process that allows each individual’s immune cells to differentiate between itself and the cells of other people or foreign organisms. A matched donor, either a relative (usually a brother or sister) or an unrelated individual from a registry, shares all 12 HLA-antigens with the patients. Historically, all allogeneic transplants have utilized matched donors because HLA mismatches were associated with high risks of GVHD.
The patient's own marrow or peripheral blood is used. Like syngeneic transplants, there is no risk of rejection or GVHD, and the GVL effect is absent. There is the additional concern of tumor contamination of the transplant. This may be a relatively minor concern for certain solid tumors but is a major problem for all blood cancers and some solid tumors. Johns Hopkins has pioneered methods to 'purge' or remove tumor cells from autologous transplants, as well as methods to reduce relapse similar to the allogeneic GVL effect.
Many patients in need of a transplant will not have a matched family member or unrelated donor. A protocol developed at the Johns Hopkins Kimmel Cancer Center in 2000 has shown that half-identical, or haploidentical, related donors can be performed with similar outcomes to that seen with matched transplants. All parents and children, and about half of siblings, are half-matches. The ability to perform haploidentical transplants has revolutionized bone marrow transplant, so that almost everyone now needing a transplant can get one. This approach, developed at Johns Hopkins, is now being studied at most of the major transplant centers around the country. Results of clinical trial were release in July 2011.
Another source of stem cells for transplantation is partially-matched cord blood. This type of transplant has become essentially standard-of-care for children needing transplantation, but because of the small size of cord blood transplants, it is still under research in adults.