• 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
  • 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
  • Endogenous antigens
  • Exogenous antigens
  • By proteosomes in the cytosol
  • By proteases in the phagosome
  • Into ER by TAP proteins
  • Into endosomes after phagocytosis
  • Directly bind to MHC I
  • Bind to MHC II after release of invariant chain
  • 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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