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Inside your bones is a thick mass of cells called bone marrow. Every hour, a small number of stem cells in it create all other kinds of blood cells that exist in your body, including leukocytes, erythrocytes, and platelets. These cells are essential to your health - leukocytes fight infection, erythrocytes carry oxygen, and platelets help the blood clot. When a person has a blood disease, such as aplastic anemia or leukemia, doctors may perform bone marrow transplants to re-establish a healthy blood supply. Many transplants occur after a patient has received chemotherapy or radiation treatment to destroy cancerous or other disease-causing cells. Both abnormal and normal cells are killed by these treatments, including stem cells. A bone marrow transplant starts the blood production process from scratch with normal stem cells. An allogeneic transplant - where another person's bone marrow is given to a patient - doesn't always work because of rejection or because of graft-versus-host disease. Rejection of the donor's marrow occurs because our bodies fight off invading foreign cells. If a donor's marrow doesn't match perfectly, the recipient's immune system may identify the new cells as foreign and destroy them, leaving the patient unable to create new blood. Graft-versus-host disease occurs because the new immune system from the donor's marrow may identify the patient's body as foreign and try to destroy it. When the donor's immune cells in the marrow attack the patient, many symptoms may result and, in severe cases, the patient could die. Doctors decrease these risks by trying to select a patient/donor pair whose immune cells will identify each other as self. An identical twin's cells will see the other twin's cells as self. But most patients do not have an identical twin. So doctors look at a person's human leukocyte antigens (HLA) to match donor and patient bone marrow. These are proteins present on the surface of our cells. They play a big role in telling immune cells that other cells are either foreign or friendly self cells. Doctors will look at HLA antigens on your siblings' cells, because you have a 25 percent chance of having an HLA match with a brother or sister. Among unrelated people, only one in 20,000 people will be an acceptable match.