What is a Bone Marrow Transplant?
A bone marrow transplant is a procedure designed to weaken or destroy tissues or cells that cause blood or immune system diseases, and then to “reset” or replace those tissues or cells to restore healthy function.
What Healthy Marrow Does
In a healthy body, the bone marrow makes young cells called stem cells that develop into mature white blood cells, red blood cells, or platelets. White blood cells fight infection. Red blood cells carry oxygen to other cells. Platelets help your blood to clot.
Blood-forming stem cells are called hematopoietic cells. “Hematopoietic” means “related to the formation of blood cells.” On average, a small number of stem cells produce an ounce of new blood, about 260 billion new cells, every day.
What Happens When this System Malfunctions
Certain diseases disrupt the function of the bone marrow. These diseases may cause the bone marrow to fail, to produce an excess of some types of blood cells or to make blood cells that don’t mature and can’t perform their normal functions. For example, in aplastic anemia, the bone marrow stops making new blood cells. In leukemia, the bone marrow makes too many immature white blood cells. In autoimmune diseases, immune system cells malfunction and attack rather than protect the body. A bone marrow transplant may help treat these diseases.
How a Transplant Works
To treat blood or immune system diseases using a transplant, doctors first give the patient chemotherapy, radiation, or both to destroy the malfunctioning cells in the body. This process is called conditioning.
- Many patients get high- or moderate-dose conditioning. It is designed to eliminate malignant cells in people with cancer and to disable their immune system, and to destroy bone marrow in people with other marrow-related diseases. High- or moderate-dose conditioning leaves patients without the ability to form new blood cells and without an immune system.
- Some patients (typically those who are older or have additional health problems) get low-dose conditioning. This treatment is designed to weaken but not destroy their bone marrow and immune system so their body will accept the donor’s stem cells.
After conditioning, doctors infuse stem cells into the patient’s blood stream through a central line, or central intravenous catheter (a small, flexible tube inserted into a large vein near the heart). The goal is for the transplanted cells to grow and develop in your bone marrow space, a process called engraftment.
- In patients who get high- or moderate-dose conditioning, engraftment means your body resumes producing blood cells and you develop an immune system again from the transplanted cells.
- In patients who get low-dose conditioning, engraftment means a new immune system develops alongside your remaining, but weakened, immune system. So for a time, you have a mixed immune system. The goal is for your new (transplanted) immune system to attack cancer cells that survived conditioning (called the graft-versus-tumor effect) and for the new immune system to eventually take over completely. This is called a mixed chimerism transplant or a mini-transplant.
Where the Transplanted Cells Come From
Stem cells used in transplants may come from any of these three sources:
- Bone marrow
- Blood circulating around the body, called peripheral blood
- Blood taken from an umbilical cord donated by a mother right after her baby’s birth, called cord blood
Different Names for Transplants
You’ll probably see transplants called by one of these names based on the source of the stem cells:
- Bone marrow transplant, or BMT
- Peripheral blood stem cell transplant, or PBSC transplant
- Cord blood transplant
- Hematopoietic stem cell transplant
The general term for all three—regardless of the source of the stem cells—is “hematopoietic cell transplant,” or HCT. The first HCTs were done using bone marrow. So many people are most familiar with the term “bone marrow transplant” and use this term regardless of the source of the stem cells. For simplicity’s sake, Seattle Cancer Care Alliance continues to refer to this treatment as bone marrow transplant.