In acute cellular rejection of a transplanted organ, which cells are the primary mediators that recognize donor antigens?

• A. Donor B lymphocytes
• B. Donor T lymphocytes
• C. Host T lymphocytes
• D. Natural killer cells

Answer: (C)

Acute cellular rejection is driven by the recipient’s T cells recognizing donor antigens. The main mediators are host T lymphocytes (CD4+ helper and CD8+ cytotoxic T cells) that react to donor MHC molecules on the transplanted tissue. This recognition can occur through direct presentation, where donor antigen-presenting cells display donor MHC to the host T cells, or through indirect presentation, where the recipient’s antigen-presenting cells present donor-derived peptides to host T cells. Activation of these T cells leads to proliferation and cytotoxic attack against the graft.

Donor B cells aren’t the primary drivers of this process in solid organ rejection, and natural killer cells may contribute to immune responses but are not the main mediators of acute cellular rejection.