CD4+ T-cell receptors recognize peptides in the context of which antigen-presenting molecule class?

Introduction / Context:
T-cell antigen recognition requires peptides presented by major histocompatibility complex (MHC) molecules. CD4+ helper T cells are restricted to antigens displayed by MHC class II, typically found on professional antigen-presenting cells (APCs) like dendritic cells, macrophages, and B cells.

Given Data / Assumptions:

• Cell subset: CD4+ T cells.
• Antigen form: peptide bound to an MHC molecule.

Concept / Approach:
MHC class II presents exogenous peptides processed in endosomal/lysosomal compartments. CD4 acts as a co-receptor by binding to non-polymorphic regions of MHC II, stabilizing the TCR–peptide–MHC interaction and facilitating signaling via the CD3 complex.

Step-by-Step Solution:

Verification / Alternative check:
APC-mediated antigen presentation to CD4+ T cells in lymph nodes is the cornerstone of helper T-cell priming.

Why Other Options Are Wrong:

• MHC I: Restricts CD8+ cytotoxic T cells.
• No MHC/Secreted receptors: Describe B-cell antibodies, not TCR recognition.
• CD1: Presents lipids to invariant NKT, not conventional CD4+ peptide recognition.

Common Pitfalls:

• Confusing the roles of CD4 and CD8 co-receptors.

Final Answer:
MHC class II molecules