A BMT is literally the transfer of a new immune system from donor to recipient to replace one that is diseased or malfunctioning. The immune system protects the body against infections and other foreign bodies which can cause malfunction of any of its systems The transfer, or transplant, takes place after the recipient’s defective immune system has been eliminated through specific regimens of high-dose chemotherapy and/or radiation.
In other words, the recipient is brought literally to the brink of death—all of his or her marrow is destroyed and no blood cells are being formed– In order for a healthy new immune system to take hold and start to function, the recipient’s body must be able to accept it, countering the natural tendency to fight off what are perceived as outside invader cells. Therein lies the importance of finding a donor immune system that is a “match” to the recipient’s.
The immune system cells which are transplanted can be taken from a donor’s bone marrow (which is extracted during a procedure done in the operating room) or from the donor’s blood stream (where the donor’s cells are collected by filtering the blood) or from the blood contained by the umbilical cord and placenta of a newborn baby right. Called “cord blood”, this cell source can be collected right after the baby’s birth. Recent scientific research indicates that use of cord blood may enable many more patients to undergo unrelated BMTs with less-precisely matched donor cells. The theory is that the less “educated” cord blood cells, which have not yet been exposed to common infections that would lead them to mount a reactive response, can be better accepted by the recipient’s body, which will not fight them off so strongly. As will be explained further in greater detail, since the factors that determine the “match” are genetically based, a patient has the best chance of finding a donor within his/her biological family. However, due to the laws of probability that govern human genetics, only 25 – 30% of patients are able to find a related (family) donor. The rest of the patients must rely on finding a donor that is unrelated to them; most likely to occur with someone of a similar ancestral genetic or ethnic background.