- What we’ll cover here
- What are stem cells and why are they transplanted?
- Why would someone need a stem cell transplant?
- Weigh the risks before transplant
- Types of stem cell transplants for treating cancer
- Sources of stem cells for transplant
- Donor matching for allogeneic transplant
- The transplant process
- Problems that may come up soon after transplant
- Transplant problems that may show up later
- Other transplant issues
- What questions should I ask my doctor before transplant?
- What’s it like to donate stem cells?
- To learn more
Donor matching for allogeneic 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 from other people as foreign, too. This becomes very important in an allogeneic stem cell transplant.
If the tissue type match between donor and recipient is not close, the patient’s immune system may see the new stem cells as foreign and destroy them. This is called graft rejection, and it can lead to graft failure. This is rare when the donor and recipient are well matched.
A more common problem is that when the donor stem cells make their own immune cells, the new 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. (See “Graft-versus-host disease” in the section called “Problems that may come up shortly after transplant” for details). The grafted stem cells attack the body of the person who got the transplant. This is another reason it’s so important to find the closest match possible.
Many factors play a role in 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 make up a person’s tissue type, which is different from a person’s blood type.
Each person has a number of pairs of HLA antigens. We inherit one of each of these pairs from each of our parents (and pass one of each pair on to each of our children). Doctors try to match these antigens when finding a donor for a person getting a stem cell transplant.
How well the donor’s and recipient’s HLA tissue types match plays a large part in whether the transplant will work. A match is better when all 6 of the known major HLA antigens are the same — a 6 out of 6 match. People with these matches have a lower chance of graft-versus-host disease, graft rejection, having a weak immune system, and getting serious infections. For bone marrow and peripheral blood stem cell transplants, sometimes a donor with a single mismatched antigen is used — a 5 out of 6 match. For cord blood transplants a perfect HLA match doesn’t seem to be as crucial for success, and even a sample with a couple of mismatched proteins may be OK.
Doctors keep learning more about better ways to match donors. Today, fewer tests may be needed, for siblings since their cells vary less than an unrelated donor. But more than the basic 6 HLA antigens are often tested on unrelated donors to reduce the risks of mismatched types. Sometimes doctors will want to look at 5 pairs of antigens, for example, to try and get a 10 out of 10 match. Certain transplant centers now require high-resolution matching, which looks more deeply into tissue types. Other centers are doing clinical trials with related half-matched donors and different chemotherapy schedules. This is an active area of research because it’s often hard to find a good HLA match.
Finding a match
There are thousands of different combinations of possible HLA tissue types. This can make it hard to find an exact match. HLA antigens are inherited from both parents. If possible, the search for a donor usually starts with the patient’s brothers and sisters (siblings), who have the same parents as the patient. The chance that any one 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 sibling is not a good match, the search could then move on to relatives who are less likely to be a good match — parents, half siblings, and extended family, such as aunts, uncles, or cousins. (Spouses are no more likely to be good matches than other people who are not related.) If no relatives are found to be a close match, the transplant team will widen the search to the general public.
As unlikely as it seems, it’s possible to find a good match with a stranger. To help with this process, the team will use transplant registries (see the “To learn more” section). Registries serve as matchmakers between patients and volunteer donors. They can search for and access millions of possible donors and hundreds of thousands of cord blood units. The largest registry in the United States is Be the Match (formerly called the National Marrow Donor Program) which has recently merged with another agency, the Caitlin Raymond International Registry. They have access to millions of international records, and have successfully matched thousands of donors and recipients.
The chances of finding an unrelated donor match improve each year, as more volunteers sign up. Today, about half of white people who need a stem cell transplant may find a perfect match among unrelated donors. This drops to about 1 out of 10 people in other ethnic groups, mostly because their HLA types are more diverse and they seem to be less likely to take part in donor registries. Depending on a person’s tissue typing, several other international registries also are available. Sometimes the best matches are found in people with a similar racial or ethnic background. Finding an unrelated donor can take months, though cord blood may be a little faster. A single match can require going through millions of records.
Now that transplant centers are more often using high-resolution tests, matching is becoming more complex. Perfect 10 out of 10 matches at that level are much harder to find. But transplant teams are also getting better at figuring out what kinds of mismatches they can get away with in which situations – that is, which mismatched sites are less likely to affect transplant success and survival.
Keep in mind that there are stages to this process – there may be several matches that look promising but don’t work out as hoped. The team and registry will keep looking for the best possible match for you. If your team finds an adult donor through a transplant registry, the registry will contact the donor to set up the final testing and donation. If your team finds matching cord blood, the registry will have the cord blood sent to your transplant center.
Last Medical Review: 10/02/2013
Last Revised: 10/02/2013