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• Lines of defense in the immune system

  • 1st line: physical & chemical surface barriers
  • 2nd line: internal cellular & chemical defense (if pathogen penetrates barriers)
  • 3rd line: immune response (if pathogen survives nonspecific, internal defenses)

  
• Nonspecific defenses


  Innate immune system (natural, not learned through experience)

  
• Specific defenses


  Adaptive immune system

  
• Components of the adaptive immune system

  • NK cells
  • Cell-mediated immunity (T cells)
  • Antibody-mediated/humoral immunity (B cells)

  
• T cells and B cells are able to develop memory
• Natural killer (NK) cells

  • Protect against viral infections & some cancers
  • Can respond very quickly compared to other lymphocytes
  • Mechanisms: destroy target cells, release interferons & cytokines to warn uninfected cells and enhance immune response

  
• Antigen


  A molecule, often on the surface of a pathogen, that the immune system recognizes as a specific threat

  
• MHC markers


  • Proteins expressed on the cell surface that display both self & non-self antigens, used in recognition of pathogens and self recognition
  • MHC-I: found on all nucleated cells
  • MHC-II: found mostly on macrophages, B cells, & dendritic cells (APCs)

  
• T cell activation


  1. Antigen presenting cell (APC) presents antigen fragment on MHC-II

  2. Helper T cell (CD4) responds by secreting cytokines that enhance immune response

  3. Initial "priming" of lymphocytes to antigen occurs in lymph tissues

  
• Cell-mediated (T cell) immune response

  1. Threat detected

  2. Macrophage presents antigen on MHC-I to activate naive cytotoxic T cell

  3. Effector cytotoxic T cell targets and kills cells displaying foreign antigen

  4. Memory T cells provide continued surveillance

  
• Antibody-mediated (B cell) immune response

  1. Naive B cell activated by effector helper T cell

  2. Plasma cell (effector B cell) secretes antibodies to neutralize foreign proteins

  3. Memory B cells provide continued surveillance