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Bone marrow consists of precursor or predecessor immature cells called stem cells. These are primitive cells that are capable of producing all types of cells.
Blood cells like RBCs, WBCs and platelets start out from young (immature) cells called hematopoietic stem cells. Stem cells mostly live in the bone marrow where they divide to make new blood cells. These cells mature into adult cells and then leave the marrow into the blood stream.
A small number of stem cells also get into the bloodstream. These are called peripheral blood stem cells.
When the bone marrow has been destroyed by disease, chemotherapy or radiation, the stem cells may be transplanted and restored. Depending on the source of the stem cells this procedure may be called:
These three types are called hematopoietic stem cell transplant. There are 3 possible sources of stem cells to use for transplants. This includes:
The first successful bone marrow transplant was done in 1968. After nearly two decades, stem cells taken from circulating (peripheral) blood were transplanted with success.
More recently, doctors have begun using cord blood from the placenta and umbilical cords of new born babies as another source of stem cells. Today nearly 50,000 new transplants are done each year.
Stem cell transplants are used to replace bone marrow that has been destroyed by:
In these conditions the stem cells are unable to make adequate blood cells. A stem cell transplant may help correct these problems.
In some cancers, such as certain leukemias, multiple myeloma, and some lymphomas, a stem cell transplant can be an important part of treatment. In these patients high doses of chemotherapy or radiation therapy is a good therapy option but cannot be given due to its potential to cause bone marrow suppression. So once high doses are used a stem cell transplant is made to replenish the suppressed marrow.
Types of bone marrow transplant include:
In this type the patient’s own bone marrow cells are taken prior to the anti-cancer procedure. These stem cells are removed, or harvested, from either bone marrow or blood and then frozen.
After this a high dose of chemotherapy or radiation therapy is given. Once the bone marrow is suppressed the frozen cells are thawed and replaced back within the body.
The advantage is that the patient gets his or her own blood cells and thus there is a decreased risk of the body’s immune system not recognizing the cells and rejecting them or mounting an attack on them. This is called graft rejection and rejection makes allogenic transplants difficult.
The disadvantage of the process is the risk of the originally taken stem cells carrying cancer cells that are reintroduced in the body to. This may bring the cancer back.
This kind of transplant is mainly used to treat some leukemias and lymphomas, and multiple myeloma. It is sometimes used for other cancers, especially in children.
In a tandem transplant, a patient gets 2 courses of high-dose chemo, each followed by a transplant of their own stem cells. All of the stem cells needed are collected before the first high-dose chemo treatment, and half of them are used for each procedure.
In this type the stem cells do not come from the patient, but from a donor whose tissue type is matched with the patient. The donor may be a family member, usually a brother or sister.
The donor may be sought from a national registry as well. This may be called a MUD (matched unrelated donor) transplant.
Cord blood transplant is another method where blood is taken from the placenta and umbilical cord of new borns. This blood has a high number of stem cells. But the number of stem cells in a unit of cord blood is often too low for large adults, so this source of stem cells has so far been used more in children.
The advantage of allogenic transplant is that the donor stem cells make their own immune cells, which may help destroy any cancer cells in the patient.
The disadvantage is the risk of graft rejection that may require lifelong use of immunity suppressing agents.
Allogeneic transplant is most often used to treat certain types of leukemia, lymphomas, and other bone marrow disorders such as myelodysplasia.
Many factors play a role in how the immune system knows the difference between “self” and “non-self.” The most important factor that is used in allogenic transplants is called human leukocyte antigen (HLA) system.
Human leukocyte antigens are proteins found on the surface of most cells. Each person has a number of pairs of HLA antigens (the best-known ones being A, B, C, DR, DQ, and DP). They inherit one of each of these antigens from each of their patents. Doctors try to match these antigens when finding a donor for a person getting a stem cell transplant.
This is possible only between identical twins or triplets with similar genetic makeup. An advantage of syngeneic stem cell transplant is that graft-versus-host disease will not be a problem. There are no cancer cells in the transplant, either, as there would be from an autologous transplant.