Related vs Unrelated Allogeneic transplantation between unrelated people can be done. But given the extraordinary complexity and diversity of HLA antigens— there are more than a billion possible combinations—finding matched but unrelated individuals is very difficult.
The National Marrow Donor Bank now operates in the United States to try to find unrelated donors. This registry has made it possible to identify donors for most patients with common HLA antigens. Yet it is still hard to find donors for patients with uncommon antigens. Adding to the problem is the uneven distribution of HLA antigens among people with different genetic backgrounds. Because most of the volunteer donors so far are of European ancestry, it is especially hard to find donors for patients with African, Asian or other ancestry. (If you are interested in participating in the National Bone Marrow Donor Program, contact your local Red Cross Blood Center for information about how to volunteer.)
Syngeneic The best donor for any bone marrow transplant is an identical twin. Since identical twins share all the same genes they have exactly the same HLA antigens, so there is no risk of rejection. This is called a syngeneic transplant and, of course, is uncommon.
Autologous A somewhat similar situation arises if someone receives his or her own bone marrow after high-dose chemotherapy and/or radiotherapy . This can be done by removing marrow before therapy and keeping it in cold storage (cryopreserving) until treatment is complete. This procedure—called an autotransplant
