The role of donor bone marrow in transplantation is to give a patient a brand-new blood and immune system. In an allogeneic transplant, healthy stem cells from a donor replace the patient’s diseased or failed marrow, then go on to make all of the patient’s red cells, white cells, and platelets for life. The donor’s immune cells can also attack remaining cancer, which is often the main reason a donor transplant is chosen.
This makes donor transplants lifesaving for many people with leukemia, lymphoma, aplastic anemia, and inherited blood disorders. It also explains the main risks, because a new immune system can attack the patient’s own tissues. Here is how the process works, from finding a donor to recovery.
What Donor Marrow Does in the Body
Donor marrow contains hematopoietic stem cells, the rare cells that produce every type of blood cell. After infusion, these cells travel to the recipient’s bones, settle into the marrow, and begin producing new cells, a process called engraftment.
Donor cells do three jobs at once:
- Restore blood production when the patient’s own marrow has failed or been cleared by treatment.
- Rebuild the immune system, which eventually becomes the donor’s immune system living in the patient.
- Fight disease through the graft-versus-leukemia effect, where donor immune cells recognize and destroy leftover cancer cells.
Normal marrow already does the first two jobs. For background on the healthy tissue being replaced, see our guide to the composition and function of bone marrow.
Who Needs a Donor Transplant?
Donor transplants are used for serious hematological and immune conditions, including:
- Acute myeloid and acute lymphoblastic leukemia, especially high-risk or relapsed disease.
- Myelodysplastic syndromes and some myeloproliferative neoplasms.
- Severe aplastic anemia and other forms of marrow failure.
- Inherited disorders such as sickle cell disease, thalassemia major, and severe combined immunodeficiency.
- Some lymphomas and other hematologic malignancies that return after other treatment.
Before transplant, patients often have signs of bone marrow failure: anemia, frequent infections, or bruising and bleeding disorders caused by low platelets.
How Donors Are Matched
Matching depends on human leukocyte antigens (HLA), proteins on cell surfaces that the immune system uses to tell self from non-self. The closer the HLA match between donor and recipient, the lower the risk of rejection and graft-versus-host disease.
HLA genes are inherited as a set from each parent, so each brother or sister has about a one in four chance of being a full match. Parents and children are usually half matches.
| Donor type | Match level | Key points |
|---|---|---|
| Matched sibling | Full HLA match | Often the first choice; available to a minority of patients |
| Matched unrelated donor | Full or near-full match | Found through volunteer registries; search takes weeks |
| Haploidentical donor | Half match | Parent, child, or sibling; widely available with modern GVHD prevention |
| Umbilical cord blood | Partial match accepted | Rapidly available; fewer cells, so slower engraftment |
Beyond HLA, doctors consider donor age, sex, blood group, and prior exposure to viruses such as cytomegalovirus when choosing between suitable donors.
How Donor Cells Are Collected
There are two main ways to collect stem cells from a donor:
- Peripheral blood stem cell collection: the donor receives growth-factor injections for several days to push stem cells into the blood, then a machine (apheresis) filters them out through a vein. This is the most common method.
- Bone marrow harvest: under anesthesia, marrow is drawn from the back of the hip bones with a needle. Donors usually go home the same or next day with some hip soreness.
Donors are screened for general health and infections. The body replaces the donated cells naturally over the following weeks.
The Transplant Process for the Recipient
First comes a full pre-transplant evaluation: heart and lung function tests, kidney and liver blood work, dental review, and infection screening. These confirm the patient can withstand treatment.
Next is conditioning, a course of chemotherapy with or without radiation. It clears diseased marrow and suppresses the immune system so the donor cells are not rejected. Reduced-intensity conditioning is gentler and allows older or less fit patients to proceed.
The donor cells are then infused through a central line. Engraftment usually occurs within two to four weeks, and the patient receives transfusions and infection prevention until counts recover.
Graft-Versus-Host Disease and Other Risks
Graft-versus-host disease (GVHD) is the defining risk of donor transplantation. Donor immune cells see the recipient’s tissues as foreign and attack them. Acute GVHD typically affects the skin (rash), gut (diarrhea), and liver (jaundice). Chronic GVHD can involve the mouth, eyes, skin, lungs, and joints.
Prevention uses immunosuppressive drugs after transplant. Treatment usually starts with corticosteroids, with other immunosuppressants or targeted drugs added if needed. Other risks include infection during immune recovery, graft failure, organ damage from conditioning, and relapse of the original disease.
When to See a Doctor
After a donor transplant, contact your transplant team immediately for fever, chills, a new rash, persistent diarrhea, yellowing of the skin or eyes, shortness of breath, or unusual bleeding. These can be early signs of infection or GVHD, and prompt treatment matters.
Frequently Asked Questions
Does the donor need to share my blood type?
No. HLA matching matters far more than ABO blood group. A mismatched blood type is managed with processing of the graft and transfusion support, and the recipient’s blood type eventually changes to the donor’s.
Is donating bone marrow dangerous for the donor?
Serious complications are rare. Donors commonly experience bone aches from growth-factor injections or hip soreness after a marrow harvest, which usually resolve within days to a couple of weeks.
Why not always use my own cells instead?
Your own cells may carry the disease, and they lack the graft-versus-leukemia effect that helps prevent relapse. For marrow failure and inherited disorders, your own stem cells are the problem, so a donor is needed.
Will my DNA change after a donor transplant?
Your blood cells will carry the donor’s DNA, while the rest of your body keeps your own. Blood tests called chimerism studies measure how much of your blood comes from the donor.
