What HLA type would T lymphocytes from a donor most likely react to in a transplant case where the donor is HLA-DR6 and the recipient is HLA-DR4?

In transplant immunology, T lymphocytes from a donor will typically recognize and react against the specific HLA (human leukocyte antigen) types present on the recipient’s cells. In this case, the donor has HLA-DR6, while the recipient has HLA-DR4.

The correct answer stems from the understanding of how T lymphocytes recognize antigens. T cells are programmed to recognize specific HLA molecules, and their reaction is primarily directed at foreign HLA types to mount an immune response. Since the donor has HLA-DR6, the T lymphocytes from the donor would not recognize their own HLA type as foreign; thus, there would not be a reaction against HLA-DR6.

The recipient, having HLA-DR4, presents a mismatched HLA type to the donor’s T cells, but these T cells are reacting to the presence of different HLA molecules, particularly any that are not found on the donor's tissues. Since the T cells in the donor do not recognize HLA-DR6 as a foreign antigen, the only relevant reaction occurs against recipient's HLA antigens that differ from the donor's.

Thus, the T cells from the donor are most likely to react specifically against