A bone marrow transplant replaces damaged or destroyed bone marrow with healthy blood-forming stem cells. These stem cells settle into the bone marrow, where they grow and begin producing new blood cells: red blood cells, white blood cells, and platelets. The procedure is used to treat cancers like leukemia and multiple myeloma, as well as serious blood disorders like sickle cell anemia and aplastic anemia.
How Blood-Forming Stem Cells Work
Bone marrow contains a tiny population of cells called hematopoietic stem cells. They make up only about 0.01 to 0.04% of all cells in the marrow, but they’re responsible for producing every type of blood cell your body needs for the rest of your life. These stem cells have two critical abilities: they can copy themselves indefinitely, and they can mature into any blood cell type.
Most of the time, these stem cells stay dormant, conserving their energy and protecting their DNA. When the body needs more blood cells (after blood loss, infection, or treatment that wipes out the marrow), they activate and begin dividing. The new cells they produce progressively specialize, eventually becoming mature red blood cells that carry oxygen, platelets that help with clotting, and the full range of white blood cells that power the immune system, including T cells, B cells, and natural killer cells.
A transplant works by introducing healthy stem cells into a patient whose own marrow is either diseased or has been intentionally destroyed as part of cancer treatment. Once infused, the new stem cells migrate to the bone marrow and begin rebuilding the blood system from scratch.
Autologous vs. Allogeneic Transplants
There are two main types of bone marrow transplant, and which one you receive depends largely on the disease being treated.
In an autologous transplant, the stem cells come from your own body. They’re collected and stored before you undergo high-dose treatment, then returned to you afterward to rebuild your marrow. This approach is used more often for lymphoma, multiple myeloma, and certain solid tumors. The advantage is that there’s no risk of your body rejecting the cells. The concern is that the collected sample could contain residual cancer cells.
In an allogeneic transplant, the stem cells come from a donor, either a family member or an unrelated volunteer. This type is used predominantly for leukemias and myelodysplastic syndromes. Donor cells carry an important benefit beyond simply rebuilding marrow: the donor’s immune cells can recognize and attack remaining cancer cells in the recipient’s body, creating a graft-versus-tumor effect. The tradeoff is a higher risk of complications, particularly the donor’s immune cells attacking the recipient’s healthy tissues.
Finding a Matched Donor
For an allogeneic transplant, the donor’s tissue type needs to closely match the recipient’s. This matching is based on proteins on cell surfaces called HLA markers. Transplant teams test four key HLA genes, each inherited in pairs, creating a possible score out of eight. An 8 out of 8 match gives the best outcomes, with higher survival rates and lower risk of complications.
When a perfect match isn’t available, a 7 out of 8 match (one mismatched marker) is generally considered acceptable. Matches below 6 out of 8 are not recommended. Siblings have about a 25% chance of being a full match. When no matched family donor exists, transplant teams search volunteer registries. If multiple equivalent donors are available, additional HLA markers beyond the core four can help prioritize the best option.
The Conditioning Phase
Before receiving new stem cells, you go through a conditioning phase. This involves high-dose chemotherapy, total body radiation, or a combination of both. The conditioning serves two purposes: it suppresses your immune system enough that your body won’t reject the incoming cells, and in cancer patients, it kills as much of the remaining disease as possible.
Radiation-based conditioning typically delivers 12 to 16 units (called gray) of total body irradiation, given in multiple smaller fractions rather than a single dose. Splitting the radiation into fractions dramatically reduces lung damage, one of the most serious side effects, from around 50% with a single dose down to about 4%. Conditioning usually lasts several days and is the most physically demanding part of the process, causing nausea, fatigue, mouth sores, and temporary hair loss.
What the Transplant Itself Looks Like
The actual transplant is surprisingly uneventful compared to what surrounds it. The stem cells are delivered through a central line or IV, similar to a blood transfusion. A typical infusion takes anywhere from 20 minutes to an hour, depending on the volume of cells and whether any side effects occur during the process. The day of infusion is designated “Day 0” on the transplant timeline.
Engraftment and Early Recovery
After infusion, the transplanted stem cells need time to find their way into the bone marrow and start producing new blood cells. This process, called engraftment, typically happens within the first 30 days. During this window, your blood counts are extremely low, leaving you highly vulnerable to infections, bleeding, and fatigue. Most patients remain in the hospital or in close outpatient follow-up during this period, often in protective isolation.
Engraftment is confirmed when blood counts begin rising on their own. White blood cells typically recover first. Once counts reach a stable threshold, the risk of life-threatening infection drops considerably, though it doesn’t disappear.
Graft-Versus-Host Disease
The most significant complication of allogeneic transplants is graft-versus-host disease, or GVHD. It happens when the donor’s immune cells identify the recipient’s tissues as foreign and attack them. It occurs in three stages: first, the conditioning regimen causes tissue inflammation; then donor immune cells activate in response to that inflammation; finally, those activated cells directly damage the recipient’s organs.
Acute GVHD typically affects three areas. The skin develops a rash or blisters. The liver becomes inflamed, sometimes causing jaundice. The gastrointestinal tract is affected with cramping abdominal pain, diarrhea, nausea, and vomiting. Patients can experience involvement in one, two, or all three of these areas. Chronic GVHD can develop later and may affect a wider range of organs over a longer period.
Long-Term Immune Recovery
Even after engraftment, rebuilding a fully functional immune system is a slow process. Innate immunity, the body’s first-line defenses, returns within the first few weeks. But the adaptive immune system, which provides targeted, long-lasting protection against specific pathogens, takes far longer.
The body’s ability to produce certain antibodies follows a rough timeline. IgM antibodies normalize around three months. IgG antibodies, which provide the bulk of long-term immunity, begin appearing between three and six months, but the ability to produce targeted IgG in response to specific threats develops gradually over one to two years. Some antibody types, particularly IgA, may remain undetectable for several years. T cells, which coordinate much of the immune response, depend on the thymus gland for development, and thymus-driven T cell recovery is extremely slow. Full T cell recovery can take two years or longer.
This extended period of immune vulnerability means transplant recipients need to take precautions against infection for many months after leaving the hospital. Vaccinations are typically re-administered on a schedule, since the new immune system doesn’t carry the memory of previous immunizations.