Which MHC class presents antigen to CD4+ T cells?

• A. Class I
• B. Class II
• C. Class III
• D. Class IV

Answer: (B)

Antigen presentation to CD4+ T cells is carried out by MHC class II molecules. These are expressed mainly on professional antigen-presenting cells such as dendritic cells, macrophages, and B cells, and they display peptides derived from extracellular proteins that have been taken up by endocytosis and processed in endosomes. The peptide–MHC II complex is recognized by the T cell receptor on CD4+ helper T cells, with the CD4 co-receptor assisting in stable binding and signaling. This pathway contrasts with MHC class I, which presents endogenous peptides to CD8+ cytotoxic T cells, and with other loci that do not present antigen to CD4+ T cells.