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The immune system plays an important role in the success of
any allogeneic stem cell transplant. The immune system normally keeps
us healthy by destroying anything in the body it sees as "foreign,"
such as bacteria or viruses. A working immune system recognizes cells
coming from other people as foreign, too.
If the tissue type match between donor and recipient is not
close, the immune system may see the new stem cells as foreign and
destroy them. This is called graft
rejection. Usually this doesn't happen because the
pre-transplant treatment (chemo and/or radiation therapy) damages the
immune system. What can happen is the donor stem cells may see the
patient's cells as foreign and turn against their new home. This type
of attack is called graft-versus-host
disease (GVHD). The grafted stem cells attack the host
recipient. To avoid this, every effort is made to find the best match
possible.
HLA matching
Many factors determine how the immune system knows the
difference between "self" and "non-self," but the most important for
transplants is the human
leukocyte antigen (HLA) system. Human leukocyte antigens
are proteins found on the surface of most cells. They determine a
person's tissue type. (This is different from a person's
blood type.)
Each person has 3 pairs of major HLA antigens (known as A, B,
and DR), for a total of 6 antigens. We inherit 1 set of A, B, and DR
antigens from each of our parents (and pass 1 of our 2 sets on to each
of our children).
How well the donor's and recipient's HLA tissue
types match plays a large part in determining whether the transplant
will be successful. The best matches are where all of the HLA antigens
are the same -- a 6 out of 6 match. These have a lower chance of
rejection or GVHD. For bone marrow and peripheral blood stem cell
transplants, often a 5 out of 6 match is acceptable, too. For cord
blood transplants a perfect HLA match doesn't seem to be as
crucial for success, and even a 4 out of 6 match may be acceptable.
Finding a match
There are literally thousands of different combinations of
possible HLA tissue types. This can make it hard to find an exact
match. Because HLA antigens are inherited, the search for a donor
usually starts with the patient's brothers and sisters (siblings), if
possible. The chance that any sibling would be a perfect match (that
is, that you both received the same set of HLA antigens from each of
your parents) is 1 out of 4.
If a good match is not found in a sibling, the search may then
move on to relatives who are less likely to be a good match -- parents
and extended family such as aunts, uncles, or cousins. (Spouses are no
more likely to be good matches than people in the general population.)
If no relatives are found to be a close match, the search widens to the
general public.
As unlikely as it seems, it is possible to find a good match
with a stranger. To help with this process, bone marrow transplant
registries are used (see the "Additional resources" section).
Registries serve as matchmakers between patients and volunteer donors.
The largest registry in the United States is the National Marrow Donor
Program. It lists the tissue types of more than 5 million volunteer
donors and about 30,000 cord blood units. Another agency, the Caitlin
Raymond International Registry, has access to millions of international
records. These agencies have successfully matched thousands of donors
and recipients. Depending on a person's tissue typing,
several other international registries also are available.
Revised: 04/21/08
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