Which antigen processing and presentation pathway is more strongly engaged by replicating attenuated viral vaccines than by nonreplicating inactivated vaccines, and why does this matter for immunity?

• A. MHC class I presentation is exclusively used by inactivated vaccines because killed particles are taken up and directly loaded onto class I molecules on the cell surface without cytosolic processing.
• B. MHC class I presentation is more strongly engaged by replicating attenuated viruses because intracellular synthesis of viral proteins leads to cytosolic antigen processing and CD8+ T cell activation, contributing to cellular immunity important for clearing infected cells.
• C. Only MHC class II pathways are relevant for both vaccine types because antigen presentation via class I cannot be triggered by vaccines under any circumstances.
• D. Neither class I nor class II presentation is relevant for vaccine response; vaccines stimulate protection only through complement activation and clotting cascades.

Answer: (B)