Transplanting any organ—a heart, a kidney, a lung or bone marrow—from one person to another is a complicated procedure. The immune system is designed to destroy foreign tissues, and "graft rejection" is always a possibility. The risk of rejection can be reduced by matching as much as possible the tissues of the donor and the tissues of the person receiving the transplant.
Depending on the availability of appropriate donors, there are three types of bone marrow transplantation that can be attempted.
Allogeneic This type involves the transfer of marrow from one individual to a completely different individual. The marrow donor is selected through a process known as HLA typing.
Matched vs Unmatched Each individual has genes on one chromosome that code for proteins that appear on cell surfaces and are involved in cell self-recognition. These bits of information, or antigens , can be identified in the laboratory. It is also becoming possible to directly compare the genes themselves. This makes it possible to tell whether bone marrow from two individuals would be "matched" or "unmatched."
Usually allogeneic transplants are between brothers and sisters. This can be done when all the known antigens are matched or if there is a mismatch at only one site. The treatment will be equally successful in either case. But, in general, the better the match, the better the outcome.
