Acute cellular rejection of a transplanted organ is primarily mediated by which cells?

• A. Host B lymphocytes recognizing donor MHC
• B. Donor T lymphocytes recognizing host MHC
• C. Host T lymphocytes recognizing donor MHC
• D. Natural killer cells recognizing donor MHC

Answer: (C)

Acute cellular rejection is driven by the recipient’s T cells responding to foreign donor MHC molecules presented in the graft. Host T lymphocytes recognize donor MHC antigens directly (direct allorecognition), become activated, and attack graft cells, leading to a lymphocytic infiltrate and tissue injury. This is distinct from antibody-mediated or graft-versus-host processes. While NK cells can contribute to immune responses, they are not the primary mediators of acute cellular rejection, and donor T cells attacking the host in a solid organ transplant would be graft-versus-host disease, not typical acute rejection.