Step 1 topics → Immunology → Antigen Processing and Presentation

Antigen Processing and Presentation

USMLE Step 1 trap: Omits the TAP transporter step required to move cytosolic peptides into the ER for MHC I loading. Cytosolic proteins are degraded by the proteasome into peptides, which are transported into the ER lumen by TAP (transporter associated with antigen processing), where they are loaded onto MHC class I before trafficking to the cell surface.

Antigen processing and presentation is how cells take proteins — either self-made or phagocytosed — and display fragments of them on MHC molecules for T cell surveillance. The two pathways (MHC I and MHC II) are mechanistically distinct, and USMLE Step 1 loves to test whether you know which cellular compartment each step happens in, which molecules are involved, and what breaks down when a component is missing. Expect both direct recall questions and vignette-style questions where a patient has a defect in TAP or HLA-DM and you have to reason through the consequence.

What makes this topic tricky is that students often blur the two pathways together, especially around where peptide loading actually occurs. The ER is central to MHC I loading, but MHC II loading happens in the endolysosome — and the whole point of the invariant chain system is to keep MHC II empty until it reaches that compartment. If you don't have that geography locked in, you'll mix up the two pathways under pressure. The TAP transporter is another classic stumbling block: peptides don't get loaded onto MHC I in the cytoplasm, they have to be actively shuttled into the ER first.

USMLE Step 1 also tests cross-presentation as a distinct concept — where dendritic cells load exogenous antigens onto MHC I to activate CD8+ T cells, which is the exception to the general rule. Understanding why each rule exists (and what the exceptions are) is how you handle the application-level questions that trip up most students.

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Common misconceptions

Common mistake
Wrong: Endogenous peptides are loaded onto MHC class I directly in the cytoplasm before entering the ER.
Right: Cytosolic proteins are degraded by the proteasome into peptides, which are transported into the ER lumen by TAP (transporter associated with antigen processing), where they are loaded onto MHC class I before trafficking to the cell surface.
Peptides generated by the proteasome in the cytoplasm cannot load onto MHC I there — MHC I molecules are assembled in the ER, not the cytoplasm. TAP (Transporter Associated with Antigen Processing) is an ATP-dependent pump embedded in the ER membrane that actively shuttles cytosolic peptides into the ER lumen, where loading onto MHC I actually occurs. If TAP is defective (as in bare lymphocyte syndrome type I), MHC I molecules can't get peptides and are degraded, leaving CD8+ T cells unable to survey intracellular proteins.
Common mistake
Wrong: MHC class II molecules are loaded with peptides in the ER alongside MHC class I.
Right: MHC class II molecules are blocked in the ER by the invariant chain (CLIP occupies the groove), trafficked to the endolysosome where CLIP is removed by HLA-DM, and then loaded with exogenous peptides from phagocytosed material.
MHC II molecules are synthesized in the ER, but they are intentionally blocked from loading peptides there — the invariant chain physically occupies the peptide-binding groove (the portion that sits in the groove is called CLIP) to prevent any ER-resident peptides from loading onto MHC II. The whole complex then traffics to the endolysosome, where proteases degrade the invariant chain and HLA-DM catalytically exchanges CLIP for the actual exogenous peptide. This compartment separation ensures MHC II only presents extracellular-derived antigens, not the same endogenous peptides displayed by MHC I.
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What the exam tests

  1. Know the sequential steps of MHC class I loading: cytosolic protein → proteasome degradation → peptide transport into the ER via TAP → loading onto MHC I → trafficking to the cell surface for CD8+ T cell recognition.
  2. Know the sequential steps of MHC class II loading: exogenous protein uptake → endosome/phagosome formation → invariant chain blocks MHC II groove in the ER → MHC II traffics to the endolysosome → HLA-DM removes CLIP → exogenous peptide loaded → surface display for CD4+ T cell recognition.

Can you avoid these mistakes?

A patient with a TAP1 gene mutation presents with recurrent bacterial infections and flow cytometry showing markedly reduced MHC I surface expression on all nucleated cells. Walk through exactly why MHC I surface expression depends on TAP function — what happens at each step when TAP is absent?
A dendritic cell phagocytoses a bacterium. Trace the bacterial protein from ingestion to display on MHC II: which compartments does it pass through, which molecules act at each step, and which T cell type ultimately recognizes it?
You're told that HLA-DM is non-functional in an experimental mouse. Predict what you would find on the surface of that mouse's antigen-presenting cells — would MHC II be absent, present but empty, or present with the wrong peptide? Explain your reasoning.
Cross-presentation allows dendritic cells to activate CD8+ T cells using exogenous antigens. Why is this the exception to the normal MHC I loading rule, and in what clinical scenario does this matter for understanding immune responses to tumors or viruses?

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