| Feature | MHC Class I | MHC Class II |
|---|---|---|
| Structure | - Chains: Heavy α chain + β₂-microglobulin (non-covalently bound). - Binding Groove: Formed by α₁ and α₂ domains. Closed-ended. - Transmembrane?: Yes, the α chain is transmembrane. | - Chains: Heterodimer of an α chain and a β chain. - Binding Groove: Formed by α₁ and β₁ domains. Open-ended. - Transmembrane?: Yes, both chains are transmembrane. |
| Antigen Source | Intracellular (viral, tumor proteins) - “Endogenous” | Extracellular (bacteria, fungi) - “Exogenous” |
| Processing | Cytosolic Proteasome | Acidified Lysosome / Phagolysosome |
| Loading Site | RER (antigen delivered via TAP) | Phagolysosome (after Invariant Chain removal) |
| Presented On | All nucleated cells + Platelets | Antigen-Presenting Cells (APCs) only |
| Presented To | CD8+ Cytotoxic T-cells | CD4+ Helper T-cells |
| Function | Kill virus-infected & tumor cells | Activate helper T-cells to coordinate immune response |
| Mnemonic | Rule of 8: MHC I x CD8 = 8 MHC II x CD4 = 8 | Rule of 8: MHC I x CD8 = 8 MHC II x CD4 = 8 |
