What Is a Transplant?

What is a Bone Marrow Transplant?

A bone marrow transplant is a procedure in which stem cells made in the bone marrow are collected, either from a donor or from the transplant patient, and stored. Then the patient receives chemotherapy, with or without radiation therapy, to attack cancer cells, the patient’s immune system, or both. The collected stem cells are then infused into the patient’s bloodstream to restart their stem-cell production and function.

What Healthy Bone Marrow Does

In a healthy body, the bone marrow makes young blood-forming cells, called hematopoietic 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. “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 Bone Marrow Transplant Works

The two main components of a transplant are conditioning and engraftment.


To treat blood, marrow, or immune-system diseases using a transplant, doctors first give the patient chemotherapy, total body irradiation, or both to destroy the malfunctioning cells in the body. This process is called conditioning.

There are two main types of conditioning: high-dose conditioning and reduced-dose conditioning.

  • High-dose conditioning: Many patients get high-dose conditioning. It is designed to eliminate malignant (cancerous) cells in people with cancer and to disable their immune system, and to destroy bone marrow in people with other marrow-related diseases. High-dose conditioning leaves patients without the ability to form new blood cells and without an immune system. This type of conditioning is called myeloablative; “myelo” refers to bone marrow, and “ablative” means to remove or destroy.
  • Reduced-dose conditioning: Some patients (typically those who are older or have additional health problems) get reduced-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 bloodstream through a central venous catheter (a small, flexible tube inserted into a large vein leading to the heart). The transplanted cells then migrate to the bone marrow to grow and develop, a process called engraftment.

Engraftment differs somewhat depending on the type of conditioning you have.

  • After high-dose conditioning: Engraftment means your body resumes producing blood cells and you develop an immune system again from the transplanted cells.
  • After reduced-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 may be called a reduced-intensity transplant, non-myeloablative transplant, or mini-transplant.

Different Names for Transplants

You may see transplants called by one of these names based on the source of the stem cells:

  • Bone marrow transplant, or BMT, which uses stems cells taken from the bone marrow
  • Peripheral blood stem cell transplant, or PBSC transplant, which uses stem cells taken from blood circulating around the body
  • Cord blood transplant, which uses stem cells collected from umbilical cord blood donated by a mother right after her baby’s birth

The general term for all three is “hematopoietic cell transplant,” or HCT. Because the first HCTs were done using bone marrow, many people are most familiar with the term “bone marrow transplant,” and they use this term regardless of the source of the stem cells. For simplicity, Seattle Cancer Care Alliance (SCCA) refers to this treatment generally as bone marrow transplant.

Diseases Treated with a Transplant

At SCCA, doctors use bone marrow transplants to treat several malignant and non-malignant diseases.

Malignant diseases treated with transplants include the following:

Non-malignant diseases treated with transplants include the following: