Antigen presentation is one of those Step 1/2 topics that’s easy to understand and still easy to mix up under time pressure—especially MHC I vs MHC II, who presents, and which T cell responds. Here’s a quick-hit mnemonic you can screenshot and reuse.
The Core Mnemonic (the one you actually remember)
“1×8 = Kill” and “2×4 = Help”
- MHC I → CD8 → cytotoxic T cells → kill infected cells
- MHC II → CD4 → helper T cells → coordinate immune response
One-liner:
MHC I shows “inside” peptides to CD8 to KILL; MHC II shows “outside” peptides to CD4 to HELP.
The Visual Device: “Inside vs Outside” Picture in Your Head
Think of a cell like a house:
- MHC I = “Inside the house” → shows what’s happening inside (endogenous proteins)
- MHC II = “Outside deliveries” → shows what came from outside (exogenous proteins)
| Feature | MHC I | MHC II |
|---|---|---|
| Antigen source | Endogenous (viral/tumor proteins made inside the cell) | Exogenous (phagocytosed proteins from outside) |
| Processed in | Proteasome → loaded in RER | Endosome/lysosome |
| Expressed on | All nucleated cells (RBCs don’t count—no nucleus) | APCs only: dendritic cells, macrophages, B cells |
| Presented to | CD8 T cells | CD4 T cells |
| Outcome | Cytotoxic killing (perforin/granzyme; apoptosis) | Helper functions (activate B cells, macrophages, CD8 responses) |
A Second Mnemonic for “Who Has MHC II?”
“DMB” = Dendritic, Macrophage, B cell
These are the professional APCs that express MHC II (and also express costimulatory molecules).
High-Yield Hooks USMLE Loves
1) Costimulation: the “Second Signal”
To activate a naive T cell, you need:
- Signal 1: TCR recognizes peptide-MHC
- Signal 2: B7 (CD80/86) on APC binds CD28 on T cell
Board favorite consequence:
If signal 1 happens without signal 2 → T cell anergy (functional inactivation).
2) Cross-presentation (a classic “wait, what?” test point)
Dendritic cells can take exogenous antigens and present them on MHC I to activate CD8 T cells.
Why it matters: kick-starts CD8 responses against viruses/tumors that don’t directly infect APCs.
3) TAP = “Transporter Associated with Processing”
For MHC I, peptides generated in the proteasome must get into the RER to load onto MHC I:
- TAP transports peptides into the RER
If TAP is defective → reduced MHC I expression → weak CD8 activation (high-yield immunodeficiency mechanism).
4) CD4 boosts macrophage killing (important in intracellular infections)
Th1 (CD4) cells activate macrophages via:
- IFN-γ (plus CD40L-CD40 interaction)
This is why CD4 deficiency (e.g., advanced HIV) predisposes to certain intracellular pathogens.
Ultra-Quick Recall Card (shareable)
- MHC I = “In” → All nucleated cells → Endogenous → CD8 → Kill
- MHC II = “Out” → DMB APCs → Exogenous → CD4 → Help
If you only remember one line:
“1×8 = Kill, 2×4 = Help.”