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Stem cell transplant

Harvested stem cells can revitalize your immune system.

Your blood cells perform a variety of functions that are critical to life, carrying oxygen and other essential elements to your tissues, removing toxins, controlling bleeding, and fighting disease as an essential part of your immune system.

Bone marrow — the spongy tissue inside certain bones — is where new blood cells are born. Your bone marrow produces hematopoietic, or blood-forming, multipotent stem cells, which divide into red cells, white cells, and platelets. As these new cells mature, they leave the marrow and circulate through the blood stream, along with a small number of the immature stem cells.

If your bone marrow is damaged, it may not be able to produce enough stem cells to create the healthy blood cells you need to support your body and fight off infection. This can happen if you develop a disease that originates in your blood or bone marrow, such as leukemia. In addition, because chemotherapy and radiation target rapidly dividing cancer cells, these treatments may destroy stem cells, which also reproduce quickly, and the bone marrow itself.

Creating a new immune system

In the 1970s, scientists developed a breakthrough procedure known as a bone marrow transplant (BMT) to treat hematologic, or blood, cancers. In this treatment, doctors administer high doses of chemotherapy to eliminate cancerous bone marrow and replace it with healthy bone marrow.

Researchers have since developed the technology to remove stem cells from the blood stream rather than having to extract bone marrow containing the cells. In a process known as a stem cell transplant (SCT), doctors administer radiation or chemotherapy to eradicate cancerous bone marrow. Then, healthy stem cells are infused into the blood where they can begin replenishing the bone marrow with new, healthy blood and immune cells. This process is known as engraftment.

Stem cell transplant was originally developed to treat blood cancers. Today doctors sometimes use the procedure to repair bone marrow and revitalize the immune systems of people with solid tumors who have undergone high doses of chemotherapy. Used this way, SCT is sometimes called stem cell rescue, and it offers hope to people whose cancer might be cured with aggressive conventional therapy.

A good match

There are two main types of stem cell transplants:

  • Autologous transplants use your own stem cells, which are removed and sometimes treated to destroy any remaining cancerous cells before they're returned to your body after you receive high-dose chemotherapy.
  • Allogeneic transplants use stem cells from a volunteer donor, often a member of your immediate family.

The success of an allogeneic transplant often depends on how closely the donor's stem cells match your own. Scientists have developed a process called HLA typing to match your genetic material and the proteins on your cells to those of a potential donor. Not surprisingly, siblings are often the closest matches.

Finding a close match is important because if the transplanted immune cells recognize your body as foreign, they may attack it, resulting in graft-versus-host disease, or GVHD, a common side effect of allogeneic SCT. Symptoms of GVHD may be relatively mild, such as skin sensitivity, but it can also cause organ damage that can be life-threatening. Chronic GVHD can be managed with immunosuppressive drugs.

However, one side effect of GVHD is actually beneficial for some people with cancer. When the transplanted immune cells attack the host's body, they also attack whatever cancer cells may remain after treatment. Known as the graft-versus-tumor (GVT) effect, this reaction may be curative in the case of slow-growing blood cancers and may reduce the risk of tumor recurrence.

Mini-transplants

Some facilities are investigating ways to make SCT milder and less toxic. One promising new method — called a non-myeloablative or mini-transplant — uses lower and less-toxic doses of chemotherapy. The lower dose destroys some, but not all, of the bone marrow. Some cancer cells may also survive the reduced chemo, but mini-transplants can trigger the GVT effect — where newly transplanted cells recognize and attack whatever cancer cells survived conventional therapy.