Bone marrow is a substance that manufactures blood components.
As in blood transfusions, the donor marrow must be the same type as the recipient's, but marrow typing is much more complicated. About 30 percent of the people who need marrow transplants have a relative, usually a brother or sister, who can donate. The remaining patients depend on finding an unrelated person with similar marrow. The likelihood of finding a match is much higher within a person's own ethnic group. Fortunately, there are national and international bone marrow registries that record the marrow characteristics of people who have agreed to be registered as potential donors.