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What are Stem Cells?

Stem cells are immature, special cells located in the bone marrow (the spongy material found inside long-bones) that mature into the three major types of blood cells:

• Red blood cells - carry oxygen to all tissues and organs of the body
• White blood cells - components of the body's immune system responsible for fighting infections
• Platelets - specialized cells in the bloodstream that are responsible for clotting of blood (stop bleeding when a person sustains a cut or an injury to blood vessels)

High-dose chemotherapy used to destroy cancer cells, unfortunately, also kills most the patient's blood-forming bone marrow and stem cells. Without these critical cells, the patient is susceptible to a variety of potentially life-threatening problems including increased susceptibility to infections and bleeding complications. Bone marrow and stem cell transplantation enables doctors to replace the critical blood-forming cells after high-dose chemotherapy to kill cancer cells has been completed.

The source of stem cells used for transplantation is either bone marrow usually harvested (removed) from the hip bone (bone marrow transplantation) or the stem cells can be obtained from the peripheral bloodstream via a procedure called apheresis (peripheral blood stem cell transplantation). In both cases, the stem cells are frozen and stored for later use until the patient has completed their course of high-dose chemotherapy and are then administered to the patient by intravenous infusion. For the purposes of this discussion the terms "bone marrow transplantation" and "stem cell transplantation" are used interchangeably.

There are two primary types of stem cell transplantation procedures:

• Allogeneic stem cell transplantation
• Autologous stem cell transplantation

Allogeneic Stem Cell Transplantation

In this procedure, the source of the stem cells used for transplantation is another person who serves as the "donor". In order to prevent complications related to rejection of the transplanted stem cells, a suitable donor must be identified whose tissue type closely matches that of the recipient. To ensure maximum success of an allogeneic transplant, the donor and recipient's tissue type must be compatible with respect to certain cell antigens or "markers" know as histocompatibility antigens (HLAs). Currently, recipient-donor compatibility for allogeneic stem cell transplantation is determined by a blood test that measures the compatibility or "match" of six different major HLA markers. The most successful allogeneic transplants are achieved in those cases where there is a "perfect match" between the donor and recipient for all six HLA markers. Successful transplants can also be achieved where only 4 or 5 HLA markers match exactly, however, the risk of complications, such as graft-versus-host-disease, is much higher. Close relatives of the patient (such as a brother or sister) are more likely to be an exact or close match than unrelated donors for allogeneic stem cell transplantation. In the event that the patient who requires a stem cell transplant has an identical twin, the twin is an ideal donor because the donor and recipient HLA markers match exactly. This type of stem cell transplant is called a syngeneic transplant.