What is an allogeneic stem cell transplant?

Blood-forming stem cells live in the bone marrow. They make red blood cells, white blood cells and platelets. In an allogeneic transplant, these stem cells come from another person, called the donor. NMDP, the US donor registry, describes it as using healthy donated cells to replace a patient’s diseased cells.

The cells are given through a central line, a tube in a large vein, much like a blood transfusion. NMDP explains that they travel to the bone marrow, where they grow into red blood cells, white blood cells and platelets.

An autologous transplant is different. The person’s own stem cells are collected and given back after strong treatment, so no donor is needed. A syngeneic transplant is a special kind of donor transplant. The American Cancer Society notes that it can only be done when a person has an identical twin or triplet.

Very high doses of chemotherapy or radiation can destroy a person’s blood-forming stem cells. The US National Cancer Institute (NCI) says a transplant restores those cells. It is used most often for blood cancers such as leukemia, lymphoma, multiple myeloma and myelodysplastic syndromes.

Donor cells can add a second benefit for some cancers. White blood cells from the donor (the graft) may attack cancer cells that are left in the body. This is called the graft-versus-leukemia or graft-versus-tumor effect, and NCI says it improves the chances that the transplant will work. It happens only when the cells come from a donor.

A transplant is also used for some blood disorders that are not cancer, such as aplastic anemia and sickle cell disease, NCI says. Here the aim is different. The US National Heart, Lung, and Blood Institute (NHLBI) explains that treatment before the transplant destroys the patient’s abnormal stem cells. The donor’s stem cells then begin making new, healthy blood cells.

The American Cancer Society notes that cells from a healthy donor are free of cancer. Unless the cells came from cord blood, the donor can also be asked to give more cells later if needed.

The transplant team starts by testing the patient’s HLA type. HLA markers are proteins found on most cells in the body. NCI explains that the more HLA markers a patient and donor share, the better the chance the body will accept the donor’s cells.

Brothers and sisters are usually tested first, because HLA type is inherited. NMDP says each full sibling has a 25% chance of being an HLA match, and that 75% of patients don’t have a fully matched donor in their own family.

If no relative is a close match, the team works with NMDP to search the registry for an unrelated volunteer donor or a stored unit of umbilical cord blood. Because HLA type is inherited, NMDP says an unrelated donor with a similar ethnic background is often the best option.

A half-matched, or haploidentical, donor shares exactly half of the patient’s HLA markers. NMDP says the donor is usually a parent, child or sibling. The American Cancer Society adds that half siblings and even cousins can be half matches.

Cord blood is collected from the umbilical cord and placenta after a baby is born. It is stored in a cord blood bank and listed on a public registry. NMDP says patients may not need to match a cord blood unit as closely as an adult donor. The American Cancer Society notes that a unit holds fewer stem cells than an adult donation, and the new cells can take longer to start making blood. Adults sometimes need more than one unit.

Newer ways to prevent graft-versus-host disease have widened the choice. A 2025 report from CIBMTR, which collects transplant data, found that the use of partly matched unrelated donors for US adults had doubled since 2020. It linked the rise to a medicine given after the transplant to prevent GVHD, called post-transplant cyclophosphamide.

Before the transplant, the patient has conditioning: chemotherapy, sometimes with radiation. NHLBI says it destroys the abnormal stem cells and weakens the immune system before the transplant. That matters because the body can reject donor stem cells.

Conditioning comes in different strengths. Standard, or myeloablative, conditioning uses high doses. Reduced-intensity conditioning uses lower doses of chemotherapy or radiation, and the American Cancer Society says this makes it useful for older people and those with other health problems. NCI calls these lower-dose donor transplants mini-transplants.

NCI says most stem cells used in transplants are collected from the bloodstream. Some donors give marrow instead, and cord blood is thawed from a bank. On transplant day, the cells drip into the central line, and the patient stays awake.

The same donor immune cells that can fight leftover cancer can also see the patient’s healthy body as foreign. This is graft-versus-host disease (GVHD). NMDP calls it a common, and sometimes serious, side effect of a donor transplant.

Acute GVHD usually starts in the first weeks and months. NMDP says chronic GVHD usually develops within a year but can appear years later. It can affect the skin, mouth, eyes, stomach and intestines, lungs and other areas, and it ranges from mild to severe and from short-lived to long-term.

Care teams try to lower the risk. NCI lists a close HLA match, medicines that calm the immune system, and removing the T cells that cause GVHD from the donated cells, called T-cell depletion. Steroids are the main treatment when GVHD does happen, NMDP says.

Infection is a serious risk early on, when white blood cell counts are low. Viruses the body had kept under control, such as cytomegalovirus (CMV), can become active again. The American Cancer Society says medicines to prevent or control infections are taken for about 6 months or longer.

The immune system is slow to rebuild. Even after blood counts return to normal, NCI says full immune recovery takes 1 to 2 years after a donor transplant. After a transplant using a person’s own cells, it takes several months.

Graft failure means the donor cells do not settle in the marrow and grow as they should. Later problems can include infertility, cataracts and, the American Cancer Society says, a higher risk of a second cancer after a donor transplant. The original disease can also come back months or years later.

An autologous transplant uses the patient’s own stored cells, so no donor is needed. The American Cancer Society notes that the body does not reject its own cells, and there is no GVHD. But there is no graft-versus-cancer effect. NCI says there is also a small risk that cancer cells are collected and given back with the healthy ones.

An allogeneic transplant brings cancer-free cells and, for some cancers, the graft-versus-leukemia effect. It also brings GVHD and a slower immune recovery. The American Cancer Society says a usual hospital stay is 3 to 4 weeks for a donor transplant and about 2 weeks for one using a person’s own cells.

NCI says the choice depends on the illness, other treatments that may work, the person’s health and whether a matching donor is available. A donor transplant is used when the patient’s own blood-forming cells are the problem, as in sickle cell disease. It is also used when the donor’s immune cells can help fight a cancer such as leukemia.

Patients don’t have to find their own donor. In the US, the transplant team works with NMDP to search for one. NMDP says it usually takes about 3 months from the start of a donor search to transplant day.

Every unrelated donor began as a volunteer who joined a registry. NMDP says more than 43 million potential donors can be searched on registries around the world. But it says registries aren’t yet diverse enough for every patient to find a full match.

In the US, NMDP asks people aged 18 to 35 to join, using a free cheek-swab kit. Other countries have their own official registries. NMDP says a volunteer could be matched with a patient anywhere in the world.