The term Bone Marrow Transplant (BMT) refers to a procedure that replaces damaged or diseased blood-forming cells with healthy ones. The more precise medical name for this therapy is Hematopoietic Stem Cell Transplantation (HSCT). This complex medical intervention is often the only path toward a cure for patients with life-threatening diseases affecting the blood and immune system.
Defining Bone Marrow Transplantation
Bone marrow is the soft, spongy tissue located inside larger bones, such as the breastbone and pelvis. It functions as the body’s primary “blood cell factory,” housing hematopoietic stem cells. These foundational cells are capable of developing into all mature blood cells, including red blood cells for oxygen transport, white blood cells for immune defense, and platelets for clotting.
HSCT is utilized to treat various serious medical conditions. These include blood cancers like leukemia and lymphoma, as well as non-cancerous conditions such as severe aplastic anemia, sickle cell disease, and specific inherited immune deficiency disorders. The goal of the procedure is to restore the body’s ability to produce a full and healthy supply of all necessary blood components.
Types of Transplants and Cell Sources
The classification of a transplant depends on the source of the replacement stem cells, resulting in two main types: autologous and allogeneic. An autologous transplant uses the patient’s own stem cells, which are collected and frozen before they undergo high-dose chemotherapy or radiation. This approach is employed to “rescue” the patient’s blood-forming system after the intensive therapy, and it carries no risk of immune rejection.
An allogeneic transplant uses stem cells donated by another person. This requires careful matching of tissue markers called Human Leukocyte Antigens (HLA). The closer the HLA match, the lower the risk that the recipient’s body will reject the new cells. The stem cells used in either type of transplant can be harvested from three sources: the bone marrow itself, peripheral blood stem cells (PBSC), or umbilical cord blood. PBSC is now the most common source for autologous transplants.
The Conditioning and Infusion Process
The transplant process begins with a conditioning regimen, involving high-dose chemotherapy, radiation, or both. This intense treatment serves two purposes: to eliminate any remaining diseased cells, such as cancer, and to suppress the patient’s existing immune system. Suppressing the immune system creates space in the bone marrow and prevents the host’s body from immediately rejecting the incoming stem cells, especially in allogeneic procedures. The intensity of this preparative regimen is classified as either myeloablative (high-dose, destroying all bone marrow) or reduced-intensity (lower-dose).
After the conditioning is complete, the patient receives the stem cell infusion, often referred to as Day Zero. This infusion is administered intravenously, similar to a regular blood transfusion, typically through a central venous catheter. The healthy stem cells enter the bloodstream and naturally migrate to the bone marrow space, a process called homing.
Monitoring Immune Integration
Following the infusion, the first major milestone is engraftment, which occurs when the transplanted stem cells settle in the bone marrow and begin to produce new, functional blood cells. This process is monitored closely, with an initial sign being the recovery of white blood cell counts, which typically takes about two to four weeks post-transplant. Successful engraftment marks the beginning of the body rebuilding its blood-forming and immune systems.
A unique complication that can arise after an allogeneic transplant is Graft-versus-Host Disease (GvHD). This occurs when the donor’s immune cells recognize the recipient’s body tissues as foreign and mount an attack, affecting organs like the skin, liver, and gastrointestinal tract. Acute GvHD usually develops within the first 100 days after the transplant, while chronic GvHD can develop later and persist for years. Complete immune recovery often requires up to a year or more.