What is the role of MHC class I and class II molecules in antigen presentation?

• A. MHC I presents endogenous peptides to CD4+ T cells; MHC II presents exogenous peptides to CD8+ T cells
• B. MHC I presents endogenous peptides to CD8+ T cells; MHC II presents exogenous peptides to CD4+ T helper cells
• C. MHC I presents exogenous peptides to CD8+ T cells; MHC II presents endogenous peptides to CD4+ T helper cells
• D. MHC I presents exogenous peptides to CD4+ T cells; MHC II presents endogenous peptides to CD8+ T cells

Answer: (B)

Antigen presentation uses two pathways that guide distinct T cell responses. MHC class I molecules, found on almost all nucleated cells, display peptides that come from proteins made inside the cell (endogenous). These are presented to CD8+ cytotoxic T cells, enabling them to recognize and kill infected or abnormal cells. MHC class II molecules are expressed mainly on professional antigen-presenting cells and present peptides from proteins that were taken up from outside the cell (exogenous) to CD4+ helper T cells. These helper cells then coordinate the immune response by activating B cells and other immune components. The described pairing aligns with these established roles: endogenous peptides with CD8+ T cells for MHC I, and exogenous peptides with CD4+ helper T cells for MHC II.