A bone marrow transplant is a complicated procedure used to restore stem cells when bone marrow has been destroyed by disease, chemotherapy, or radiation. Hematopoietic stem cells (not to be confused with stem cells from embryos) refer to the immature cells in the body that make blood. They are found in the bone marrow, where they divide to make new blood cells. Once the blood cells are mature, they leave the bone marrow and enter the bloodstream.
A bone marrow transplant can be a key part of treatment for some types of leukemia, multiple myeloma, and some lymphomas, among other diseases. Those diseases respond best to high doses of chemotherapy or radiation, but the high doses can destroy the bone marrow's ability to make blood cells. So in many cases, high doses of treatment are given to kill the cancer, and then the patient receives a stem cell transplant (through an injection into the vein) to replenish the blood cells.
Transplants generally use autologous or allogeneic cells. In an autologous transplant, a patient's own marrow cells are harvested and then re-injected, thus eliminating the risk of rejection. In an allogeneic transplant, the cells come from someone else, often a relative, or a donor found through a national registry, such as the National Marrow Donor Program. It is vital that the donor's tissue type matches that of the recipient.
In either method, the extracted stem cells can come from three sources: bone marrow, the bloodstream (peripheral blood), and umbilical cord blood from newborns.
In a transplant involving bone marrow, doctors normally extract the marrow from a donor's pelvis, which has a rich supply of stem cells. This treatment requires general anesthesia for the donor.
In peripheral blood collection, donors are given a growth factor – a hormone-like substance, administered a few days before the harvest. This causes the stem cells to grow faster and move from the bone marrow into the blood. Today, peripheral blood is used more often than bone marrow because it is easier to extract, resulting in faster recovery for the donor.
Cord blood is primarily used for transplants in children, as the amount of stem cells is often too small for use in large adults. It is taken from the placenta and umbilical cord of newborns; the chord and placenta would otherwise be discarded after birth.
Transplant recipients undergo lengthy preparation and extensive aftercare, including: tests, medication, and weeks or months of recovery. For older or weaker transplant recipients, whose health conditions make it risky to wipe out all of the bone marrow before a transplant, doctors are experimenting with a type of "mini-transplant." This procedure uses less chemo and/or radiation, allowing both donor and patient cells to co-exist in the patient's body.
Any type of transplant carries serious risk and bone marrow transplants are no different. Complications can range from nausea to infection, bleeding, heart or lung problems – and in extreme cases – death. Although there is no risk of rejection in autologous transplants, there is often no improvement in an immune system that was weak to begin with. Allogeneic stem cells carry the risk that the recipient's body will reject the new stem cells – or that the donor cells will attack the patient's body. This condition is known as graft-versus-host disease, which can be mitigated with immunosuppressive drugs.
The decision to transplant is not made lightly. But bone marrow transplants offer hope of resuming a "normal" life to many patients. Today hundreds of thousands of patients have had stem cell transplants, and up to 50,000 new transplants are done each year.