A stem cell transplant is a procedure that replaces defective or damaged cells in patients whose normal blood cells have been affected by cancer.Stem cell transplants commonly are used to treat leukemia and lymphoma, cancers that affect the blood and lymphatic system. They also can help patients recover from or better tolerate cancer treatment.

In addition, these stem cell transplants are used to treat hereditary blood disorders, such as sickle cell anemia, and autoimmune diseases, such as multiple sclerosis.

Stem cell transplants use hematopoieticstem cells. These immature cells begin life in the bone marrow and eventually develop into the various types of mature blood cells, including:

There are two types of stem cell transplantation:

Cells are harvested from the patient's own bone marrow before chemotherapy and are replaced after cancer treatment. These are used most often to treat diseases like lymphoma and myeloma. They have little to no risk of rejection or graft versus host disease (GVHD) and are therefore safer than allogeneictransplants.

Stem cells come from a donor whose tissue most closely matches the patient.These cells can also come from umbilical cord blood extracted from the placenta after birth and saved in special cord blood banks for future use. MDAnderson's Cord Blood Bank actively seeks donations of umbilical cords.

Allogeneic transplants are often used to treat diseases that involve bone marrow, such as leukemia. Unlike autologous transplants, they generate a new immune system response to fight cancer. Their downside is an increased risk of rejection or GVHD.

Stem cell transplant patients are matched with eligible donors by human leukocyte antigen (HLA) typing. HLA are proteins that exist on the surface of most cells in the body. HLA markers help the body distinguish normal cells from foreign cells, such as cancer cells.

HLA typing is done with a patient blood sample, which is then compared with samples from a family member or a donor registry. It can sometimes take several weeks or longer to find a suitable donor.

The closest possible match between the HLA markers of the donor and the patient reduces the risk of the body rejecting the new stem cells (graft versus host disease).

The best match is usually a first degree relative (children, siblings or parents). These can be full matches or half-match related transplants, also known as haploidentical transplants.However, about 75% of patients do not have a suitable donor in their family and require cells from matched unrelated donors (MUD), who are located through registries such as the National Marrow Donor Program.

Because the patients immune system is wiped out before a stem cell transplant, it takes about six months to a year for the immune system to recover and start producing healthy new blood cells. Transplant patients are at increased risk for infections during this time, and must take precautions. Other side effects include:

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Stem Cell Transplants | MD Anderson Cancer Center