Extracellular antigens are typically presented to CD4+ T cells by which mechanism?

• A. MHC class II molecules presenting peptides derived from internalized proteins
• B. MHC class I molecules presenting peptides from cytosolic proteins
• C. B cell receptor presentation
• D. Natural killer cell presentation

Answer: (A)

Extracellular antigens are captured by antigen-presenting cells through endocytosis or phagocytosis, then degraded in endosomal compartments. The resulting peptides are loaded onto MHC class II molecules and presented on the cell surface, where they are recognized by CD4+ helper T cells. This pathway contrasts with MHC class I, which presents peptides from proteins made inside the cell to CD8+ cytotoxic T cells. B cells can contribute to this process by internalizing antigen via their B cell receptor and presenting peptides on MHC II to helper T cells. Natural killer cells don’t present extracellular antigens to CD4+ T cells via MHC II.