What is the purpose of the invariant chain in antigen processing associated with class II MHC molecules?

• A. Helps transport peptides to the binding site
• B. Blocks binding of endogenous peptides
• C. Binds to CD8+ T cells
• D. Cleaves peptides into the proper size for binding

Answer: (B)

Invariant chain acts as a protective chaperone for MHC class II molecules. Its main job is to occupy the peptide-binding groove in the immature MHC II complex, preventing endogenous (self) peptides from binding in the endoplasmic reticulum. This ensures that loading of peptides happens later, in the endosomal/lysosomal pathway where exogenous antigens are processed. The invariant chain also directs the MHC II molecule to these vesicular compartments and is degraded to leave a placeholder (CLIP) that is then exchanged for the actual antigen peptide with the help of specific accessory molecules.

So, the purpose being to block binding of endogenous peptides explains why this molecule is crucial for presenting externally derived peptides to CD4+ T cells.