What happens in bone marrow transplants? Why is it difficult to find a match?
The concept of bone marrow transplants (BMTs) is remarkably simple, but the procedure itself has risks. It must be carried out with a great deal of precision and caution.
Bone marrow transplants have long been used to treat leukemia, certain kinds of lymphomas and aplastic anemia. In the last decade, great strides have been made with BMTs, and they are now more widely used in conjunction with radiation or chemotherapy treatment for additional types of cancer and a few other diseases. The cancer treatment is given at such high doses that it kills off the bone marrow along with the cancer cells.
All blood cells are manufactured in the bone marrow, which is spongy tissue located in pockets inside our bones. In a bone marrow transplant, the marrow is replaced with marrow from a relative, an unrelated person or from the transplant recipient, which is called an autologous transplant.
When the donor is a relative, the BMT is called allogeneic. If the relative is an identical twin, the marrow is an exact genetic match — a syngeneic transplant. When the donor is no relation, it’s known as an unrelated allogeneic transplant.
The procedure is much more like a transfusion than the surgery you might imagine when you hear the word transplant. Typically, the marrow is extracted from the donor’s hip with a needle. It is then injected into the veins of the recipient, just like a blood transfusion. The bone marrow eventually travels back into the bones and starts functioning again.
One of the jobs of bone marrow is creating the white blood cells, which are the main constituent of the body’s immune system. Before and after the BMT, patients essentially have no white blood cells and therefore no defense against infection. The patient must be protected from any possibility of exposure to infection for several weeks until the bone marrow is up and running and the white blood cell count has increased.
The difficulty in finding a match arises from the body’s own immune defenses. Bone marrow tissue contains certain genetic characteristics called HLAs. The recipient’s bone marrow must closely match the donor’s. Otherwise the body might reject the donation, at least initially, causing a very dangerous disease called graft-versus-host-disease.
The chances of finding a good match are highest with full brothers or sisters, who inherited their HLAs from the same pool of four possible genes in their parents. Close matches can also be found in unrelated donors, particularly if the recipient has HLA genes that are more common.