CD4+ T-cell receptors recognize peptides in the context of which antigen-presenting molecule class?
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ANo MHC required; free peptides only
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BMHC class I molecules
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CMHC class II molecules
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DSecreted receptors that bind soluble antigens
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ECD1 molecules presenting lipids only
Answer
Correct Answer: MHC class II molecules
Explanation
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:
Step 1: Recall restriction: CD4+ → MHC II; CD8+ → MHC I.Step 2: Eliminate options: “no MHC required” is false for αβ TCRs; MHC I is for CD8+; secreted receptors refers to antibodies, not TCR; CD1 specializes in lipids and NKT cells, not general CD4+ peptide recognition.Step 3: Select MHC class II.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