Two weeks after an allogeneic transplant, a patient develops a erythematous maculopapular rash, diarrhea, and elevated liver enzymes. What is the most likely mediator of these symptoms?

• A. Donor T lymphocytes
• B. Host T lymphocytes
• C. B cells
• D. Immune complexes

Answer: (A)

Acute graft-versus-host disease after an allogeneic transplant is driven by donor T lymphocytes attacking host tissues because of differences in MHC antigens. Donor CD4+ and CD8+ T cells become activated by the recipient’s antigen-presenting cells and target organs rich in recipient antigens, especially the skin, gastrointestinal tract, and liver. The resulting inflammation causes the characteristic erythematous maculopapular rash, diarrhea, and elevated liver enzymes. B cells and immune complexes are not the primary mediators in this scenario, and host T cells would more likely mediate rejection of the graft rather than GVHD.