Bone marrow transplant donors in the recipients must be matched for which antigen system?

The correct answer is HLA. Human Leukocyte Antigen (HLA) matching is crucial in bone marrow transplantation because it plays a vital role in the immune response. HLA molecules are involved in the recognition of self versus non-self by the immune system. When a donor's HLA type closely matches that of the recipient, the risk of graft-versus-host disease (GVHD) is significantly reduced, improving the chances for successful engraftment and overall transplant outcomes.

In the context of bone marrow transplantation, HLA compatibility implies that the donor and recipient share similar HLA alleles, which helps ensure that the donor's immune cells do not perceive the recipient's body tissues as foreign. This antigen matching is far more critical than the other listed antigen systems in the context of hematopoietic stem cell transplantation. While blood group compatibility (ABO) is also important, it is secondary to HLA matching regarding overall transplant success and safety.