A blood stem cell transplant replaces damaged or destroyed blood-forming cells with healthy ones. It’s most commonly used to treat blood cancers like leukemia and lymphoma, but it also treats certain immune disorders, bone marrow failure, and even autoimmune diseases like multiple sclerosis. The procedure involves wiping out your existing bone marrow with chemotherapy or radiation, then infusing new stem cells that rebuild your blood and immune system from scratch.
You might also hear this called a bone marrow transplant or hematopoietic stem cell transplant. These terms refer to the same basic concept, though the source of the stem cells and the specific approach can vary significantly.
How It Works
Blood stem cells are the parent cells that produce every type of blood cell in your body: red blood cells that carry oxygen, white blood cells that fight infection, and platelets that help your blood clot. These stem cells live mainly in your bone marrow, the spongy tissue inside your bones. When disease damages these cells or causes them to grow out of control, a transplant can replace them with a healthy supply.
The basic idea is straightforward, but the process is intensive. It unfolds in three main phases: conditioning, infusion, and engraftment.
The Three Phases of the Procedure
Conditioning
Before new stem cells can take hold, your existing bone marrow needs to be cleared out. This step, called conditioning, uses high-dose chemotherapy, radiation, or both. It serves multiple purposes: destroying cancerous cells if cancer is the reason for the transplant, suppressing your immune system so it won’t reject the new cells, and creating space in your bone marrow for the incoming stem cells to settle.
Conditioning intensity varies. Some regimens are strong enough to permanently destroy your marrow’s ability to produce blood cells on its own, making the transplant essential for survival. Others are less intense, called reduced-intensity conditioning, which causes significant but potentially reversible drops in blood cell counts. The lightest regimens cause only minimal drops and are designed mainly to suppress the immune system enough for the new cells to establish themselves. Your medical team selects the intensity based on your age, overall health, and the disease being treated.
Infusion (Day Zero)
The transplant itself is surprisingly simple compared to everything leading up to it. The stem cells are delivered through an IV line, much like a blood transfusion. A typical infusion of peripheral blood stem cells takes 20 minutes to an hour. This day is called “Day Zero” in transplant timelines, with all other days counted before or after it.
During the infusion, you might experience flushing, hives, nausea, stomach cramps, or changes in blood pressure and heart rate. Chest tightness, fever, and chills are also common. Your nursing team monitors you closely and can treat most of these side effects as they arise.
Engraftment
After infusion, the stem cells travel through your bloodstream and find their way into your bone marrow, where they begin producing new blood cells. This process, called engraftment, typically takes two to four weeks. During this waiting period, your blood cell counts are critically low, leaving you extremely vulnerable to infections and bleeding. Most patients remain hospitalized or in close medical supervision during this phase.
Types of Transplants
There are two main categories, and the distinction matters because it affects everything from who qualifies to what risks you’ll face.
An autologous transplant uses your own stem cells. They’re collected from your blood before conditioning, frozen, then returned to you after chemotherapy destroys your marrow. Because the cells are yours, your body won’t reject them, and there’s no risk of graft-versus-host disease (more on that below). The trade-off is that your own cells could potentially contain lingering cancer cells, which raises the chance of relapse. This type is commonly used for multiple myeloma and certain lymphomas.
An allogeneic transplant uses cells from a donor: a sibling, an unrelated volunteer, or sometimes a parent or child. The advantage here is a powerful side benefit. The donor’s immune cells can recognize and attack any remaining cancer cells in your body, an effect called graft-versus-tumor. However, those same donor immune cells can also attack your healthy tissues, causing graft-versus-host disease. Allogeneic transplants carry higher treatment-related risks but may offer a stronger defense against relapse in aggressive cancers.
A third, rare type is a syngeneic transplant, which uses cells from an identical twin. This eliminates the risk of immune rejection entirely but also loses the graft-versus-tumor benefit.
Where the Stem Cells Come From
Stem cells can be harvested from three sources, each with distinct characteristics.
- Peripheral blood: The most common source today. The donor receives injections over several days that coax stem cells out of the bone marrow and into the bloodstream, where they’re filtered out through a process similar to blood donation.
- Bone marrow: Stem cells are drawn directly from the hip bone with a needle under anesthesia. This was the original method and is still used in certain situations.
- Cord blood: Stem cells collected from the umbilical cord and placenta after a baby is born. Cord blood cells are more immature, which means they don’t need to be as precisely matched to the recipient. However, platelet recovery is significantly slower after cord blood transplants compared to the other sources, because these cells follow a different maturation pathway.
Finding a Donor Match
For allogeneic transplants, the donor’s immune markers need to closely match yours. These markers, called HLA (human leukocyte antigens), are proteins on the surface of your cells that your immune system uses to distinguish “self” from “foreign.” The closer the match, the lower the risk of complications.
A fully matched sibling is the ideal donor, matched at six key HLA markers. For unrelated donors, the standard is matching at eight markers (HLA-A, B, C, and DRB1) using high-resolution DNA testing. Siblings have roughly a 25% chance of being a full match.
When no fully matched donor is available, a haploidentical donor may be an option. This is a family member, often a parent or child, who shares only half of your HLA markers. Advances in transplant techniques have made these half-matched transplants increasingly viable, expanding the pool of potential donors considerably.
Conditions Treated
Blood stem cell transplants treat a wide range of diseases. Blood cancers are the most common reason: leukemia, lymphoma, and multiple myeloma. The transplant may be used as a first-line treatment or as a salvage option when cancer returns after initial therapy.
Non-cancerous conditions also respond to transplantation. These include aplastic anemia (where the marrow stops producing enough blood cells), thalassemia and sickle cell disease (inherited blood disorders), and myelofibrosis (scarring of the bone marrow). Increasingly, autologous transplants are being used for severe autoimmune diseases, particularly relapsing-remitting multiple sclerosis, where high-dose chemotherapy followed by a stem cell “reset” can halt disease progression.
Graft-Versus-Host Disease
GVHD is the most significant complication unique to allogeneic transplants. It happens when the donor’s immune cells identify your tissues as foreign and attack them. The risk doesn’t exist with autologous transplants since the cells are your own.
Acute GVHD typically develops within the first 100 days after transplant and most commonly targets three organs. The skin is affected in about 70% of cases, usually starting as an itchy or painful rash on the palms, soles, and shoulders that can spread across the body. The gastrointestinal tract is involved in roughly 74% of cases, causing watery diarrhea that persists even without eating, along with abdominal pain, nausea, and mouth sores. The liver is affected in about 44% of cases, usually alongside skin or gut symptoms rather than on its own.
Chronic GVHD develops after 100 days and can affect nearly any organ system. It tends to resemble autoimmune conditions, with dry eyes, skin changes, joint stiffness, and lung problems being common features. Chronic GVHD can persist for months or years and significantly affects quality of life.
Recovery and Immune Rebuilding
Day 100 after transplant is considered a major milestone. The risk of transplant-related complications and serious infections is highest during those first 100 days, when your immune system is at its weakest. During this period, your body is especially vulnerable to viral infections, including reactivation of viruses like CMV and Epstein-Barr that may have been dormant in your system.
Your innate immune system, the broad first line of defense, generally recovers within the first several months. But your adaptive immune system, the targeted branch that remembers specific threats, takes one to two years to fully rebuild. During that extended recovery, you’ll likely need to avoid crowds and certain environments, follow specific dietary precautions, and receive revaccinations since your previous immunities are wiped out by conditioning.
Who Qualifies for a Transplant
Not everyone with a treatable condition is a candidate. Transplant teams evaluate your fitness using a standardized set of criteria: heart function (specifically how well the left ventricle pumps), lung capacity, kidney function, liver function, and mental health. Your overall physical performance, how well you can carry out daily activities, plays a significant role in the assessment.
A scoring system called the HCT Comorbidity Index evaluates the severity of existing health problems across 17 organ systems. The goal is to determine whether your body can withstand the intensity of conditioning and the prolonged immune suppression that follows. Age is a factor but not an automatic disqualifier. Transplants are now performed in patients over 70, though outcomes in this group show a one-year survival rate around 55% for allogeneic transplants, reflecting the higher toll on older bodies.
Survival and Outcomes
Outcomes vary enormously depending on the disease being treated, the type of transplant, and the patient’s overall health. Autologous transplants generally carry lower short-term risks, with treatment-related mortality around 7% at three years in studies of lymphoma patients. Allogeneic transplants carry higher treatment-related mortality, around 32% at three years in similar studies, largely because of GVHD and infection risks. But for aggressive cancers with high relapse rates, the graft-versus-tumor effect of an allogeneic transplant can make the difference between long-term remission and disease recurrence.
Improvements in donor matching, conditioning regimens, infection prevention, and GVHD management have steadily improved survival rates over the past two decades. The choice between transplant types, and whether to pursue a transplant at all, depends on balancing the risks of the procedure against the risks of the underlying disease.