Which molecule presents exogenous peptides to CD4+ helper T cells?

• A. CD40
• B. TCR
• C. MHC class II
• D. B7

Answer: (C)

Exogenous antigens are presented to CD4+ helper T cells via MHC class II molecules on professional antigen-presenting cells. After extracellular proteins are taken up, they’re processed in endosomes/lysosomes and the resulting peptides are loaded onto MHC class II, then displayed on the surface for recognition by the T cell receptor on CD4+ T cells. This peptide-MHC II presentation is what activates helper functions that coordinate humoral and cellular immunity. CD40, TCR, and B7 are involved in other aspects of T cell activation or signaling but do not present exogenous peptides themselves; CD40 is a receptor on APCs, the TCR is the T cell receptor that recognizes peptide-MHC, and B7 provides costimulation to T cells. The presenting molecule for exogenous peptides to CD4+ T cells is MHC class II.