Antigen Processing

Aka: Antigen Processing, Antigen Presentation, Endocytic Pathway, Endocytic Processing and Presentation of Exogenous Antigen, Cytosolic Pathway, Cytosolic Processing and Presentation of Endogenous Antigen, Presentation of Peptide Antigen, Antigen, Antigen Presenting Cell, Antigen-Presenting Cell, Hapten, Epitope

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II. Definitions

  1. Antigen
    1. Molecules recognized by antibodies, or by T Cells (as peptides presented via MHC complex on host cells)
    2. Possible Antigens include Proteins, Nucleic Acids, lipids, complex Carbohydrates
    3. Antigens must be macromolecules, large enough to generate an immune response (Immunogens)
    4. Antigens have one or more Epitopes to which immune agents (e.g. antibodies) bind
  2. Hapten
    1. Antigen that is too small on its own to generate an immune response
    2. When Haptens are attached to macromolecules (immunogens), that may act as Epitopes for immune binding
  3. Immunogen
    1. Antigen that is large enough to generate an immune response
  4. Epitopes (Determinants)
    1. Antigen regions to which immune agents (e.g. antibodies) bind
  5. Antigen Processing
    1. Conversion to peptides of exogenous Antigens (endocytic path) and endogenous Antigens (cytosolic path)
    2. Peptide form is required for T-Cell recognition

III. Images

  1. T-Cell Activation
    1. TCellActivation.jpg

IV. Type: Cytosolic Pathway for Endogenous Antigens (MHC Class I Complex)

  1. Indications
    1. Endogenous Antigens (e.g. Intracellular Bacteria, intracellular Parasites, viruses)
    2. Antigen Processing starts within the cell cytosol
  2. Pathway
    1. Intracellular Antigens within the cell cytosol are processed by proteolytic enzymes within proteasome
      1. Processing results in converting Antigens to peptides
    2. Peptides are moved to endoplasmic reticulum where they are bound to MHC Class I molecules
    3. Antigen Peptide-MHC Class I Complex is moved to cell surface via golgi complex
  3. Antigen Presenting Cells (APC or Host Cells) displaying MHC Class I Complex
    1. All nucleated host cells (unlike MHC Class II of the Endocytic Pathway)
  4. Cells with Receptors Recognizing MHC Class I Complex
    1. CD8+ T-Cells (Cytotoxic T-Cells)

V. Type: Endocytic Pathway for Exogenous Antigens (MHC Class II Complex)

  1. Indications
    1. Exogenous Antigens
  2. Pathway
    1. Exogenous Antigens are engulfed by cells via Phagocytosis (endocytosis) as Phagosomes (endosomes)
      1. Cell membrane invaginates around the Antigens, surrounding them in a Vesicle
    2. Phagosomes fuse with Lysosomes (forming phagolysosomes)
      1. Proteolytic enzymes within the Lysosome breakdown or process the Antigen into peptides
    3. Antigenic peptides are bound to MHC Class IIMolecules while still inside the phagolysosome
    4. Antigen Peptide-MHC Class II Complex is moved to cell surface via secretory Vesicles
  3. Antigen Presenting Cells (APC or Host Cells) displaying MHC Class II Complex
    1. Langerhans Cells and Dendritic Cells
    2. Macrophages
    3. B-Cell (B-Lymphocyte)
  4. Cells with Receptors Recognizing MHC Class II Complex
    1. CD4+ T-Cells (T-Helper Cells)
      1. BCellActivation.jpg

VI. References

  1. Mahmoudi (2014) Immunology Made Ridiculously Simple, MedMaster, Miami, FL
  2. Guyton and Hall (2006) Medical Physiology, p. 419-50

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