Hematopoietic Cell Transplantation (HCT) is a medical procedure that replaces a person’s damaged bone marrow with healthy, blood-forming stem cells. The bone marrow is the body’s factory for producing blood cells, and HCT provides new stem cells to restart the production of red blood cells, white blood cells, and platelets. This process is often compared to rebooting a computer for the body’s blood and immune functions.
During the procedure, specialized stem cells are infused intravenously. These new cells travel through the bloodstream to the bone marrow, where they begin to grow and produce healthy blood components. This replenishes the body’s blood supply and can reconstruct the immune system. HCT is a complex treatment reserved for specific life-threatening diseases.
Conditions Treated by HCT
Hematopoietic cell transplantation is a treatment for a range of diseases affecting the blood and immune system. It is most frequently used for cancers of the blood or bone marrow, such as leukemia, multiple myeloma, and some types of lymphoma. In these cases, the transplant makes it possible for patients to receive high doses of chemotherapy or radiation to eliminate cancer cells. The subsequent infusion of healthy stem cells then restores the bone marrow’s ability to produce blood, which is destroyed by the aggressive cancer treatment.
HCT also treats bone marrow failure syndromes. Aplastic anemia, a condition where the body stops producing enough new blood cells, is one such disorder treated with HCT. For these patients, the transplant provides a new set of stem cells to repopulate the bone marrow and restore normal blood cell production, correcting the underlying defect.
HCT is also an option for certain inherited or genetic disorders. These include conditions that affect the immune system, such as severe combined immunodeficiency (SCID), and those impacting red blood cells, like sickle cell disease or thalassemia. In these situations, the transplant replaces the patient’s defective stem cells with healthy donor cells, providing a foundation for a functional immune system or the ability to produce normal blood cells.
Sources and Types of Stem Cells
The stem cells used in a transplant are categorized based on their origin, which determines the type of transplant a patient receives. The decision is influenced by the patient’s disease, age, health, and the availability of a suitable donor.
One category is an autologous transplant, where stem cells are collected from the patient’s own body. These cells are harvested from the blood or bone marrow, often when the disease is in remission, and are then cryopreserved for later use. This method is used to support patients undergoing high-dose chemotherapy for cancers like lymphoma and multiple myeloma. A primary benefit is that the risk of rejection is eliminated because the patient is their own donor.
In contrast, an allogeneic transplant uses stem cells from another person, a donor. This donor can be a relative, an unrelated volunteer, or in a syngeneic transplant, the patient’s identical twin. For allogeneic transplants to be successful, the donor and recipient must have closely matched human leukocyte antigens (HLA). HLA are proteins on cell surfaces that the immune system uses to recognize which cells belong in the body. A close HLA match reduces the risk of rejection and a complication known as graft-versus-host disease.
The hematopoietic stem cells are collected from one of three sources:
- Bone marrow harvesting was the original method and involves extracting marrow directly from the back of the donor’s hip bones under anesthesia.
- Peripheral blood stem cell (PBSC) collection is more common and involves giving the donor medication that stimulates the bone marrow to release extra stem cells into the bloodstream. These are then collected through a process similar to donating plasma.
- Umbilical cord blood is collected from the umbilical cord and placenta after a baby is born and stored in cord blood banks. Cord blood units contain fewer stem cells, making them suitable for children, and they do not need as perfect an HLA match.
The Patient’s Transplant Journey
The patient’s path through HCT is a structured process that unfolds in phases over weeks to months. The journey begins with the conditioning regimen, which prepares the body for the new stem cells. This phase involves administering high-dose chemotherapy, and sometimes total body irradiation, over several days. The goals of conditioning are to eliminate any remaining cancer cells and to suppress the patient’s immune system to prevent it from rejecting the new donor cells.
Following conditioning is the infusion day, often called the patient’s “new birthday.” The collected stem cells are infused into the patient’s bloodstream through a central venous catheter. The procedure is similar to a blood transfusion, is not a surgical operation, and takes one to five hours.
Once infused, the stem cells travel to the bone marrow, where engraftment begins. This is a waiting period of two to four weeks, during which the new stem cells take hold and begin producing new blood cells. During this time, the patient is highly vulnerable to infection because their white blood cell counts are extremely low. Patients remain in the hospital in a protected environment and receive supportive care, including blood transfusions and antibiotics.
A concern during the early post-transplant period for allogeneic transplants is acute graft-versus-host disease (GVHD). This condition occurs when the donor’s immune cells (the graft) recognize the patient’s body (the host) as foreign and attack its tissues. The most common signs are skin rashes, diarrhea, and liver problems. Patients are given medications to help prevent GVHD, but it can still occur and may require treatment with steroids.
Life After Transplant
The recovery process following HCT extends long after the initial hospital stay, taking months or even years. The immune system requires a significant amount of time to rebuild its function. For patients with an autologous transplant, immune recovery can take several months. For those who receive allogeneic transplants, the timeline is longer, typically one to two years, as the new immune system learns to function.
During this recovery, patients remain under close medical supervision with frequent follow-up appointments to monitor blood counts and overall health. A necessary step in rebuilding immunity involves revaccination. The conditioning therapy erases the immunological memory from previous childhood vaccinations. Patients must be revaccinated against diseases like measles, mumps, polio, and tetanus, with a schedule tailored to their recovery.
For recipients of allogeneic transplants, a long-term consideration is chronic graft-versus-host disease (GVHD). Unlike acute GVHD, the chronic form can develop months or years later. This condition can affect various parts of the body, including the skin, mouth, eyes, joints, and internal organs, with symptoms ranging from mild to severe. Managing chronic GVHD often requires long-term treatment with immunosuppressive medications.
Beyond the physical challenges, the journey after transplant involves emotional and psychological adjustments. Patients and their families navigate the stress of long-term recovery, the uncertainty of outcomes, and resuming daily activities. Support from healthcare teams, family, and peer groups can be a valued part of returning to a new sense of normalcy.