Finding a Stem Cell Donor
If you plan to get an allogeneic (allo) transplant, you need someone to donate stem cells or bone marrow. This person (the donor) might be a close relative, or they might be unrelated to you.
If possible, it’s very important that you and your donor are a close tissue match. Your transplant team will work with you to find a closely matched donor.
Why is it important to closely match donors?
This helps avoid graft rejection. It also makes graft-versus-host disease (GVHD) less likely. Or if GVHD does happen, the symptoms might be mild.
Graft rejection
Graft rejection happens when the recipient’s immune system sees the donor cells as strange and tries to destroy them, as it would with bacteria or a virus.
Graft rejection can lead to graft failure, but this is rare when the donor and recipient are well matched.
Graft-versus-host disease
When the donor stem cells make their own new immune cells, those immune cells may see the cells of the recipient as strange, and they may not welcome their new “home.”
This is called graft-versus-host disease (GVHD). Though there have been major improvements to prevent GVHD, it is still a risk. Your transplant team will work to lower this risk by finding a fitting donor.
See Stem Cell Transplant Side Effects for more on this.
What makes a donor a match?
Many things play a role in how your immune system knows the difference between itself and something that doesn’t belong. For transplants, the most important factor is the human leukocyte antigen (HLA) system.
HLAs 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 multiple pairs of HLA antigens. We inherit them from both of our parents, and we pass them on to our children. A transplant team tries to match these antigens when they look for a donor for a stem cell transplant.
What is HLA matching?
How well the donor and recipient’s HLA tissue types match plays a large role in whether the transplant works. Most transplant centers require high-resolution (Hi-Res) matching. Hi-Res matching looks more deeply into tissue types and allows for more specific HLA matching.
Doctors keep finding better ways to match donors. But to reduce the risks of mismatched types between unrelated donors, more than the basic 6 HLA antigens are tested.
Doctors often try for a perfect 8/8 match, but HLA typing may look for 10/10 or 12/12 match. Even if a perfect match is not available, fitting donors are now available to all people who need a transplant.
People with these matches have a lower chance of:
- Graft-versus-host disease
- Graft rejection
- Weakened immune system
- Serious infections
If a fitting full HLA match isn’t available, a half-matched or partially matched (also called mismatched) donor could be a good option.
A perfect match is even less important for cord blood transplants. In fact, a certain amount of mismatching helps ensure a stronger graft-versus-cancer effect.
Your transplant team will consider your donor options, prescribe medication for GVHD prevention, and monitor you closely for complications.
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 a person’s brothers and sisters (full siblings).
A perfect match with a sibling means you both got the same set of HLA antigens from each of your parents. The chance of a perfect match with any one of your siblings is 1 out of 4.
If a sibling is not a full match, the search can move on to relatives who are less likely to be a full match but might be a half match (haploidentical):
- Parents
- Half siblings
- Extended family (aunts, uncles, cousins)
Finding an unrelated donor
If no relatives are a close match, the transplant team widens its search to the general public through a search registry. A search registry includes people who’ve had HLA testing and volunteered as donors. The search registry finds donors who are the highest possible match.
As unlikely as it seems, it’s possible to find a full match with someone who isn’t related to you. To help with this process, the transplant team will use transplant registries to serve as matchmakers between you and volunteer donors.
They can search for and access millions of possible donors and hundreds of thousands of cord blood units.
NMDP
Toll-free number: 1-888-999-6743
Website: https://www.nmdp.org/
Blood & Marrow Transplant Information Network
Toll-free number: 1-888-597-7674
Website: www.bmtinfonet.org
Depending on your tissue typing, several other international registries are also available. Sometimes, the best matches are found in people with a similar racial or ethnic background.
When compared to other ethnic groups, White people have a better chance of finding a perfect match for stem cell transplant among unrelated donors. This is because ethnic groups have differing HLA types, and in the past, there were fewer non-White donors in registries.
However, the chances of finding an unrelated donor improve each year as more volunteers sign up for registries. Improvements in transplant care have made a perfect match less critical to good outcomes.
How long does it take to find an unrelated donor?
Finding an unrelated donor can take your transplant team anywhere from a few weeks to a few months. Finding cord blood might be faster. Finding and deciding between possible donors takes time, and not all donors will be available to donate when called.
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 transplant team and registry will keep looking for the best possible match for you.
If your transplant team finds:
- An adult donor through a transplant registry: The registry will contact the donor to set up the final testing and donation.
- Matching cord blood: The registry will have the cord blood sent to your transplant center.
Learn more
- Written by
- References
Developed by the American Cancer Society medical and editorial content team with medical review and contribution by the American Society of Clinical Oncology (ASCO).
American Society of Clinical Oncology (ASCO). What is a stem cell transplant (bone marrow transplant)? Accessed at cancer.net. Content is no longer available.
Lv M, Gorin NC, Huang XJ. A vision for the future of allogeneic hematopoietic stem cell transplantation in the next decade. Sci Bull. 2022 Oct 15;67(19):1921-4. Accessed at https://doi.org/10.1016/j.scib.2022.09.004 on March 3, 2025.
Negrin RS. Donor selection for hematopoietic cell transplantation. Accessed at Donor selection for hematopoietic cell transplantation - UpToDate on March 12, 2025.
Stephen R. Spellman; Hematology 2022—what is complete HLA match in 2022?. Hematology Am Soc Hematol Educ Program 2022; 2022 (1): 83–89. https://doi.org/10.1182/hematology.2022000326
Vaillant AA, Modi P, Mohammadi O. Graft-Versus-Host Disease. [Updated 2024 Jun 7]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK538235/
Last Revised: July 10, 2025
American Cancer Society medical information is copyrighted material. For reprint requests, please see our Content Usage Policy.
American Cancer Society Emails
Sign up to stay up-to-date with news, valuable information, and ways to get involved with the American Cancer Society.
