Bone marrow, the center of large bones, is the place where new blood cellss are produced. Bone marrow contains stem cells which produce three types of blood cells: leukocytes, red blood cells, and platelets.
Bone marrow can be transplanted. It is extracted from the largest bones, typically the sternum or the hip, of a HLA-compatible donor by a large needle that reaches the center of the bone and is transfused like blood into the patient.
A compatible donor is found by doing blood tests on potential donors. Perfect matches, the donor matches on all six HLA types result in the most successful transplants. Less perfect matches, four or five matches, can also result in successful transplants but the risk of complications is greater.