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Antigen Processing and Presentation

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Topic updated on 05/24/17 4:09pm

Introduction
  • Antigens must be presented to the adaptive immune system so that
    • specialized antigen presenting cells (APCs) can activate the immune system
    • killer T-cells can monitor the intracellular contents of all cells
    • helper T-cells can be alerted to both intracellular and extracellular antigens
  • Two types of antigens are processed by cells for presentation on the cell surface
    • endogenous antigens are proteins produced by the cell
    • exogenous antigens are proteins that are taken up by the cell
  • Both types are linked to major histocompatability complexes (MHC) during processing so that
    • they can be stably exported to the cell surface
    • they can be recognized specifically by T-cells
  • Antigens are presented to two distinct cell populations including
    • CD4+ helper T-cells that
      • recognize antigens loaded onto MHC class II
    • CD8+ killer T-cells that
      • recognize antigens loaded onto MHC class I
Antigen Processing
  • Antigens must undergo a number of transformations before being presented including
    • degradation of proteins into component peptides
    • translocation of the peptides into the appropriate compartment
    • loading of the peptides onto MHC proteins
  • The processing of antigens must therefore preserve information about
    • the location from where the antigen originated
    • the type of response that is required
Antigen Processing Pathways
Feature MHC Class I MHC Class II
Function
  • Allow for sampling of intracellular antigens
  • Signal that a cell is infected or abnormal
  • Allow for sampling of extracellular antigens
  • Signal that pathogens are within the host
Target cell
  • CD8+ killer T-cells
  • Rule of 8: (MHC) 1 x (CD) 8 = 8
  • CD4+ helper T-cells
  • Rule of 8: (MHC) 2 x (CD) 4 = 8
Antigens
  • Endogenous antigens
  • Exogenous antigens
Degradation
  • By proteosomes in the cytosol
  • By proteases in the phagosome
Translocation
  • Into ER by TAP proteins
  • Into endosomes after phagocytosis
Loading
  • Directly bind to MHC I
  • Bind to MHC II after release of invariant chain
Defect
  • Absent CD8+ activity
  • Absent CD4+ activity
 
Antigen Presentation
  • After processing, antigenic peptides are loaded on surface MHC proteins where
    • T-cell receptors can bind specifically to the peptide
    • CD4 and CD8 can bind specifically to the corresponding MHC
    • Integrins can bind APCs
  • Together, this set of interactions allows for
    • specific detection of antigens
    • activation of T-cells

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Qbank (1 Questions)

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(M1.IM.24) Antigen presentation of extracellular pathogens by antigen presenting cells requires endocytosis of the antigen, followed by the degradation in the acidic environment of the formed phagolysosome. Should the phagolysosome become unable to lower its pH, what is the most likely consequence? Topic Review Topic

1. Deficient NK cell activation
2. Deficient presentation of pathogens to CD4 T-cells
3. Deficient presentation of pathogens to CD8 T-cells
4. Deficient cell extravasation
5. Deficient expression of B7

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