Which MHC molecule is necessary for antigen recognition by CD4+ T cells?

• A. Class II
• B. Class I
• C. Class III
• D. No MHC molecule is necessary.

Answer: (A)

CD4+ T cells recognize antigens only when they are displayed by MHC class II molecules on antigen-presenting cells. The CD4 co-receptor helps stabilize the interaction between the T-cell receptor and the peptide–MHC II complex, enabling activation. MHC II presents peptides from extracellular proteins that are taken up by endocytosis and processed in endosomes, then loaded onto the MHC II molecule for presentation. In contrast, MHC class I presents endogenous peptides to CD8+ T cells, and MHC class III encodes components not used for presenting antigens to T cells. Therefore, MHC class II is the molecule required for antigen recognition by CD4+ T cells.