Final answer:
MHC class I molecules rely on TAP for peptide loading of antigens, generally derived from cytosolic proteins, while MHC class II molecules associate with the invariant chain (li) chaperone to ensure proper antigen processing and presentation on antigen-presenting cells such as dendritic cells, macrophages, and B cells.
Step-by-step explanation:
The student is asking about the antigen processing and presentation mechanisms associated with MHC class I and MHC class II molecules. MHC class I relies on the transporter associated with antigen processing (TAP) for peptide loading. The proteasome degrades cytosolic proteins, including those from intracellular pathogens such as viruses, and the fragments are transported into the endoplasmic reticulum by TAP where they meet MHC class I molecules.
On the other hand, MHC class II molecules interact with the invariant chain (li) chaperone. The invariant chain facilitates proper folding of the MHC class II molecule and prevents premature loading of peptides by occupying the peptide-binding groove. It is expressed on cells such as dendritic cells, macrophages, and B cells, which serve as antigen-presenting cells (APC). This invariant chain is later replaced by peptides from extracellular antigens that have been processed within the cell.