Explain antigen presentation and MHC
What is antigen presentation?
The two classes — the core distinction
The whole subtopic reduces to one 80/20 idea: there are two display systems, one for "inside" threats and one for "outside" threats.
HOW MHC Class I works (endogenous pathway)
Derive the pathway by asking: how does a cytosolic protein reach the surface groove?
- Source: Normal turnover proteins + any viral proteins are made in the cytosol.
- Chopping: The proteasome degrades these proteins into short peptides (~8–10 aa).
- Why 8–10? The MHC I groove is closed at both ends, so it clamps a short peptide like a hot-dog with both bun-ends sealed.
- Transport: Peptides are pumped into the ER by the transporter TAP (Transporter associated with Antigen Processing).
- Why the ER? MHC I is assembled and folded in the ER — the peptide must meet it there.
- Loading: Peptide slots into newly-made MHC I (helped by chaperones like tapasin).
- Display: The loaded MHC I travels via Golgi → surface, where CD8⁺ T-cells scan it.
HOW MHC Class II works (exogenous pathway)
- Source: Extracellular microbes/proteins are taken in by phagocytosis/endocytosis into a vesicle (phagosome/endosome).
- Chopping: The vesicle fuses with a lysosome; acidic proteases cut the protein into peptides (~13–25 aa, longer because the class II groove is open at both ends).
- MHC II protection: Class II is made in the ER but its groove is blocked by the invariant chain (Ii) so cytosolic peptides don't get in.
- Why? This keeps the two pathways separate — class II must wait for endosomal peptides.
- Loading: In the vesicle, invariant chain is degraded down to CLIP, then HLA-DM swaps CLIP out for the antigenic peptide.
- Display: MHC II–peptide goes to the surface, scanned by CD4⁺ helper T-cells.
Cross-presentation (the exception worth knowing)
Common mistakes (Steel-manned)
Recall Feynman: explain to a 12-year-old
Imagine every cell in your body is a little shop. Robber-catchers (T-cells) patrol outside but can't go in. So each shop must put a display window at the front showing tiny samples of everything it's cooking inside. If a shop is secretly cooking something evil (a virus), the sample in the window gives it away, and a catcher smashes that shop. The display window is MHC, the samples are peptides, and showing them is antigen presentation. Shops that also eat things from the street have a second window (MHC II) to show what they ate, so a manager cell (helper T) can organise the whole street's defence.
Flashcards
What two things does an MHC molecule bind together?
Which MHC class presents endogenous (intracellular) antigens?
Which T-cells recognise MHC class I?
Which MHC class presents exogenous antigens and to which T-cells?
State the "product rule" pairing MHC class with CD co-receptor.
Which cells express MHC class I?
Which cells express MHC class II?
What organelle chops proteins for the MHC I pathway?
What transporter moves peptides into the ER for MHC I loading?
Why are MHC I peptides short (~8–10 aa)?
Why are MHC II peptides longer (~13–25 aa)?
What blocks the MHC II groove until it reaches the endosome?
What is cross-presentation?
What is the human name for MHC genes?
Why is MHC highly polymorphic?
T-cell epitopes are linear, antibody epitopes can be...?
Connections
- T-cells and T-cell Receptors
- CD4 Helper T-cells and CD8 Cytotoxic T-cells
- Dendritic Cells and Antigen-Presenting Cells
- Innate vs Adaptive Immunity
- Proteasome and Protein Degradation
- HLA and Transplant Rejection
- B-cells and Antibody Production
- Autoimmunity and Self-Tolerance
Concept Map
Hinglish (regional understanding)
Intuition Hinglish mein samjho
Dekho, basic idea ye hai: aapki har cell ke andar kya ho raha hai, wo bahar se dikhta nahi. Toh immune system ka killer T-cell andar ghus ke check nahi kar sakta. Isliye har cell apne andar bante proteins ke chote tukde (peptides) ko surface pe MHC molecule ke andar rakh ke "display" karti hai. Isi display ko bolte hain antigen presentation — jaise dukaan apni window me sample rakhti hai.
Do system hain. MHC class I cell ke andar wale (endogenous) proteins dikhata hai — jaise virus ke proteins. Ye saari nucleated cells pe hota hai, aur ise CD8+ killer T-cell dekhta hai. Agar viral peptide dikha, toh killer us infected cell ko maar deta hai. MHC class II cell ne jo bahar se khaya (exogenous, jaise bacteria) uske tukde dikhata hai — ye sirf professional APCs (dendritic cell, macrophage, B-cell) pe hota hai, aur ise CD4+ helper T-cell dekhta hai jo poori defence coordinate karta hai.
Yaad rakhne ka simple trick: class × CD = 8. MHC I ke saath CD8, MHC II ke saath CD4. Aur "One = Inside, Two = ouTside". Pathway bhi logic se yaad rahega: MHC I me cytosol ka protein → proteasome kaatta hai → TAP ER me bhejta hai → MHC I load hoke surface pe. MHC II me vesicle ka protein → lysosome kaatta hai → invariant chain (CLIP) hatta hai → peptide load hota hai.
Ye topic important isliye hai kyunki isi se decide hota hai ki body virus ko andar se pakde ya bahar wale germs ko. Transplant reject hone ka reason bhi yahi MHC (HLA) hai — isliye donor match karna padta hai. Exam me pathway ke steps + "kaun class kis T-cell ko" ye 80% marks dila dega.