When the healthy stem cells come from you, the procedure is called an autologous transplant. When the stem cells come from another person, called a donor, it is an allogeneic transplant. Blood or bone marrow transplants most commonly are used to treat blood cancers or other kinds of blood diseases that decrease the number of healthy blood cells in the body. These transplants also may be used to treat other disorders.

For allogeneic transplants, your doctor will try to find a donor whose blood cells are the best match for you. Your doctor will consider using cells from your close family members, from people who are not related to you and who have registered with the National Marrow Donor Program, or from publicly stored umbilical cord blood. Although it is best to find a donor who is an exact match to you, new transplant procedures are making it possible to use donors who are not an exact match.

Blood or bone marrow transplants are usually performed in a hospital. Often, you must stay in the hospital for one to two weeks before the transplant to prepare. During this time, you will have a narrow tube placed in one of your large veins. You may be given medicine to make you sleepy for this procedure. You also will receive special medicines and possibly radiation to destroy your abnormal stem cells and to weaken your immune system so that it wont reject the donor cells after the transplant.

On the day of the transplant, you will be awake and may get medicine to relax you during the procedure. The stem cells will be given to you through the narrow tube in your vein. The stem cells will travel through your blood to your bone marrow, where they will begin making new healthy blood cells.

After the transplant, your doctor will check your blood counts every day to see if new blood cells have started to grow in your bone marrow. Depending on the type of transplant, you may be able to leave, but stay near the hospital, or you may need to remain in the hospital for weeks or months. The length of time will depend on how your immune system is recovering and whether or not the transplanted cells stay in your body. Before you leave the hospital, the doctors will give you detailed instructions that you must follow to prevent infection and other complications. Your doctor will keep monitoring your recovery, possibly for up to oneyear.

Although blood or bone marrow transplant is an effective treatment for some conditions, the procedure can cause early or late complications. The required medicines and radiation can cause nausea, vomiting, diarrhea, tiredness, mouth sores, skin rashes, hair loss, or liver damage. These treatments also can weaken your immune system and increase your risk for infection. Some people may experience a serious complication called graft-versus-host disease if the donated stem cells attack the body. Other people may reject the donor stem cells after the transplant, which can be an extremely serious complication.

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The rest is here:
Blood and Bone Marrow Transplant - NHLBI, NIH