Noesis
4.7.8 · HinglishImmune System

Explain antigen presentation and MHC

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4.7.8 · Biology › Immune System

Antigen presentation kya hai?

Do classes — core distinction

Poora subtopic ek 80/20 idea pe aata hai: do display systems hain, ek "andar" ke threats ke liye aur ek "bahar" ke threats ke liye.

Figure — Explain antigen presentation and MHC

MHC Class I KAISE kaam karta hai (endogenous pathway)

Pathway derive karo ye poochh ke: ek cytosolic protein surface groove tak kaise pahunchti hai?

  1. Source: Normal turnover proteins + koi bhi viral proteins cytosol mein banti hain.
  2. Chopping: Proteasome in proteins ko short peptides (~8–10 aa) mein degrade karta hai.
    • 8–10 kyun? MHC I groove dono ends pe band hoti hai, isliye woh ek short peptide ko hot-dog ki tarah clamp karti hai jisme dono bun-ends seal hote hain.
  3. Transport: Peptides ER mein pump hote hain transporter TAP (Transporter associated with Antigen Processing) ke zariye.
    • ER kyun? MHC I ER mein hi assemble aur fold hota hai — peptide ko wahan usse milna zaroori hai.
  4. Loading: Peptide naye bane MHC I mein slot karta hai (chaperones jaise tapasin ki madad se).
  5. Display: Loaded MHC I Golgi ke zariye surface tak jaata hai, jahan CD8⁺ T-cells use scan karti hain.

MHC Class II KAISE kaam karta hai (exogenous pathway)

  1. Source: Extracellular microbes/proteins phagocytosis/endocytosis ke zariye vesicle (phagosome/endosome) mein le jaaye jaate hain.
  2. Chopping: Vesicle lysosome ke saath fuse hoti hai; acidic proteases protein ko peptides (~13–25 aa, zyada lamba kyunki class II groove dono ends pe open hoti hai) mein kaatte hain.
  3. MHC II protection: Class II ER mein banta hai lekin uski groove invariant chain (Ii) se block rehti hai taaki cytosolic peptides andar na jaayein.
    • Kyun? Yeh dono pathways ko alag rakhta hai — class II ko endosomal peptides ka intezaar karna hota hai.
  4. Loading: Vesicle mein, invariant chain CLIP tak degrade hoti hai, phir HLA-DM CLIP ko antigenic peptide se swap kar deta hai.
  5. Display: MHC II–peptide surface pe jaata hai, jahan CD4⁺ helper T-cells use scan karti hain.

Cross-presentation (jaan'ne laayak exception)

Common mistakes (Steel-manned)

Recall Feynman: ek 12-saal ke bachhe ko samjhao

Socho tumhare body ki har cell ek chhoti si dukaan hai. Robber-catchers (T-cells) bahar patrol karte hain lekin andar nahi ja sakte. Isliye har dukaan ko apne saamne ek display window rakhni hoti hai jo dikhaye ki andar kya bana raha hai. Agar koi dukaan secretly kuch bura bana rahi hai (ek virus), toh window mein sample reveal kar deta hai, aur ek catcher us dukaan ko tod deta hai. Display window hai MHC, samples hain peptides, aur unhe dikhana hai antigen presentation. Woh dukaanein jo street se bhi cheezein khaati hain unke paas ek doosri window (MHC II) hoti hai jo dikhaye ki unhone kya khaya, taaki ek manager cell (helper T) poori street ki defence organize kar sake.

Flashcards

MHC molecule kin do cheezein ko saath bind karta hai?
Ek short peptide (antigen fragment) ko, aur use T-cell receptor ke saamne present karta hai.
Kaun sa MHC class endogenous (intracellular) antigens present karta hai?
MHC class I.
Kaun si T-cells MHC class I ko recognize karti hain?
CD8⁺ cytotoxic (killer) T-cells.
Kaun sa MHC class exogenous antigens present karta hai aur kaun si T-cells ko?
MHC class II, CD4⁺ helper T-cells ko.
MHC class ko CD co-receptor se pair karne wala "product rule" batao.
MHC class × CD number = 8 (I↔CD8, II↔CD4).
Kaun si cells MHC class I express karti hain?
Saari nucleated cells.
Kaun si cells MHC class II express karti hain?
Professional APCs — dendritic cells, macrophages, B-cells.
MHC I pathway ke liye proteins kaun sa organelle kaatta hai?
Proteasome (cytosol mein).
Kaun sa transporter MHC I loading ke liye peptides ko ER mein le jaata hai?
TAP (Transporter associated with Antigen Processing).
MHC I peptides chhote (~8–10 aa) kyun hote hain?
Class I groove dono ends pe band hoti hai, isliye ek short peptide ko clamp karti hai.
MHC II peptides zyada lamba (~13–25 aa) kyun hote hain?
Class II groove dono ends pe open hoti hai, isliye peptides bahar hang kar sakte hain.
MHC II groove ko endosome tak pahunchne se pehle kaun block karta hai?
Invariant chain (CLIP tak degrade hoti hai, phir HLA-DM swap karta hai).
Cross-presentation kya hai?
Dendritic cells ka exogenous antigen ko MHC I pe route karna taaki CD8⁺ T-cells prime ho sakein.
MHC genes ka human naam kya hai?
HLA (Human Leukocyte Antigen).
MHC highly polymorphic kyun hai?
Taaki ek population peptides ki bahut badi diversity present kar sake aur kai pathogens se resist kar sake.
T-cell epitopes linear hote hain, antibody epitopes ho sakte hain...?
Conformational (3-D folded shape pe depend karte hain).

Connections

  • T-cells and T-cell Receptors
  • CD4 Helper T-cells aur 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

chops proteins into

bound in groove of

inspected by

splits into

splits into

degraded by

feeds peptides to

presents to

on all nucleated cells

loaded onto

presents to

expressed on

Antigen Presentation

MHC / HLA molecule

Short peptides

T-cells

MHC Class I

MHC Class II

Endogenous cytosolic proteins

Exogenous swallowed proteins

CD8 cytotoxic T-cells

CD4 helper T-cells

Proteasome

Professional APCs only