Antigen receptors on T lymphocytes bind HLA class II plus peptide complexes with the help of which accessory molecule?

• A. CD2
• B. CD3
• C. CD4
• D. CD8

Answer: (C)

The key idea is how T cells recognize peptide presented by MHC molecules and the role of a co-receptor in that recognition. For antigen-presenting cells displaying peptide on HLA class II, the helper T cell uses CD4 as the accessory molecule. CD4 binds to the nonpolymorphic regions of MHC class II and helps stabilize the TCR–peptide–MHC interaction while bringing the kinase Lck to the TCR–CD3 signaling complex, lowering the activation threshold. The TCR–CD3 complex is responsible for signaling after engagement, but it does not by itself bind MHC class II with the same stabilizing effect as CD4. CD8 binds MHC class I and is used by cytotoxic T cells, not by class II recognition. CD2 is an adhesion molecule that enhances cell contact but does not confer specificity for MHC class II binding.