Who is the best match for a stem cell transplant?

Most often, the best match for an allogeneic (donor) stem cell transplant is a brother or sister.

To decide whether a donor's stem cells are a match, they are tested for their HLAs, which stands for human leukocyte antigens. HLAs are sets of proteins, or markers, found on most cells in your body. Each person has a different set of HLAs.

The more HLAs that you and the donor have in common, the better the chance that your body will accept the donor's stem cells. Because siblings share more of these markers than unrelated people on average, a brother or sister is often the best match.

A donor can also be someone who is not related, if their cells are a close enough match. Matching closely matters because it lowers the risk that your immune system will see the transplanted cells as foreign, and it lowers the risk of graft-versus-host disease.