Which co-receptor is required for CD4 positive T cells to recognize antigen presented by HLA class II?

Which co-receptor is required for CD4 positive T cells to recognize antigen presented by HLA class II?

• A. CD2
• B. CD4
• C. CD3
• D. CD8

Answer: (B)

CD4 acts as the co-receptor for CD4+ T cells recognizing antigens presented by MHC class II. It binds to non-polymorphic regions of MHC class II, stabilizing the TCR–peptide–MHC interaction and bringing the kinase Lck to the TCR–CD3 complex to initiate signaling. This co-receptor function is essential for proper activation of helper T cells by peptides shown on professional antigen-presenting cells. CD3 is part of the TCR signaling machinery but doesn’t define MHC restriction; CD8 is the co-receptor for recognition of peptides presented by MHC class I, and CD2 is an adhesion molecule without the same role in MHC class II–restricted activation.

CD4 acts as the co-receptor for CD4+ T cells recognizing antigens presented by MHC class II. It binds to non-polymorphic regions of MHC class II, stabilizing the TCR–peptide–MHC interaction and bringing the kinase Lck to the TCR–CD3 complex to initiate signaling. This co-receptor function is essential for proper activation of helper T cells by peptides shown on professional antigen-presenting cells.

CD3 is part of the TCR signaling machinery but doesn’t define MHC restriction; CD8 is the co-receptor for recognition of peptides presented by MHC class I, and CD2 is an adhesion molecule without the same role in MHC class II–restricted activation.