An allogenic transplant refers to a bone marrow transplant when stem cells come from a relative or unrelated donor. A bone marrow transplant may stop the spread of cancer cells and enable the patient's immune system to recover its ability to function. Stem cells from a sibling present the best chance of a match between the donor and the recipient, but unrelated persons may share sufficient similarities for a successful bone marrow transplant.
Human leukocyte antigen (HLA) testing determines whether the donor in an allogeneic transplant is a suitable candidate for the surgery. Humans inherit six antigens at birth, three from each parent. Not all six antigens need to match between the allogeneic transplant donor and the patient, but the risk of rejection lessens with the number of matching antigens. If bone marrow cannot be taken from a relative, bone marrow registry programs can sometimes find a donor.
Three main types of bone marrow transplants are used. In addition to the allogeneic transplant, an autologous method is available for some patients. Autologous means the stem cells are taken from the patient's blood, which increases the odds of success. This method is commonly used to treat tumors. A syngeneic transplant occurs when bone marrow is donated by an identical twin.
In an allogeneic transplant, the actual bone marrow can be harvested or blood can be drawn from the donor. Called a peripheral blood stem cell transplant, blood from the donor is transferred to the patient in a procedure similar to a blood transfusion. Healthy stem cells in the donor blood travel to the bone marrow and begin growing to produce new cells and platelets.
Bone marrow is found in the large bones in the body, and consists of adult cells that differ from embryonic stem cells found in umbilical cords. The marrow breeds an environment where stem cells grow into white blood cells and platelets. The cells and platelets are stored in bone marrow until the body needs them to fight infection or disease.
The success of an allogeneic transplant is divided into three phases. Stem cells from a healthy donor are tested to determine the likelihood of a successful transplant. Donor cells are extracted during surgery from various bones in the donor's body. In the meantime, the recipient undergoes massive doses of chemotherapy that destroy his or her immune system and stem cells that are non-functioning. Donor cells are grafted into the patient, where they begin growing and produce new white and red blood cells in two to four weeks.
Bone marrow transplants are commonly used to treat leukemia and other cancers, along with blood diseases and compromised immune system disorders. In an allogeneic transplant, it typically takes one to two years before the recipient's immune system is fully functioning. If the patient's own stem cells are used, complete recovery is possible within a few months. Frequent blood tests tell the physician if the patient is producing healthy new cells and platelets.
Complications of an allogeneic transplant include the risk of infection or bleeding from the high level of chemotherapy used to kill the patient's immune system. Infertility and organ damage are also possible. Sometimes donor cells begin attacking the patient's organs, or the patient's body rejects the implanted stem cells. Drugs can be administered to fight infection, while a blood transfusion may reverse anemia after a transplant.