MHC class I and II molecules and the antigen processing and presentation pathways tested on USMLE Step 1.
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- What is the human gene locus that encodes MHC molecules called?
- The human leukocyte antigen (HLA) complex
- On which human chromosome is the HLA complex located?
- Chromosome 6
- What cell types express MHC class I molecules?
- Nearly all nucleated cells
- What cell types express MHC class II molecules under normal conditions?
- Professional antigen presenting cells: dendritic cells, macrophages, and B cells
- What T cell type recognizes antigen presented on MHC class I?
- CD8+ cytotoxic T cells
- What T cell type recognizes antigen presented on MHC class II?
- CD4+ helper T cells
- What are the three classic MHC class I gene loci in humans?
- HLA-A, HLA-B, and HLA-C
- What are the three classic MHC class II gene loci in humans?
- HLA-DP, HLA-DQ, and HLA-DR
- What is the source of peptides typically presented on MHC class I?
- Endogenous (cytosolic) proteins, including viral proteins synthesized within the cell
- What is the source of peptides typically presented on MHC class II?
- Exogenous proteins taken up from outside the cell via phagocytosis or endocytosis
- What proteolytic complex degrades cytosolic proteins into peptides for MHC class I loading?
- The proteasome
- What transporter moves peptides from the cytosol into the endoplasmic reticulum for loading onto MHC class I?
- TAP (transporter associated with antigen processing)
- Where does MHC class I peptide loading occur?
- In the endoplasmic reticulum
- What cellular compartment degrades exogenous proteins into peptides for MHC class II loading?
- The phagolysosome (endosomal/lysosomal compartment)
- What invariant chain-derived peptide occupies the MHC class II peptide groove before final antigen loading, and what removes it?
- CLIP (class II-associated invariant chain peptide), which is removed by HLA-DM to allow antigenic peptide loading
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