28- immune evasion

• Ways Immunity Can Fail

  • Primary Immunodeficiency Diseases
  • Secondary Immunodeficiency's
  • Evasion and Subversion by pathogens
• Pathogen's Problem

  • Need to propagate in host and spread to others
  • Needs to grow and evolve quick enough to avoid activating a large response, but not to much or it will kill the host
  • Ideally should not elicit a response
• Antigenic Variation

  • Extracellular pathogens
  • Serotypes: Different versions of the same pathogen with different surface proteins
  • Person will initially be infected with one, create antibodies for it but then could be reinfected with a different serotype and then would need to create all new antibodies
• Antigenic Shift

  • Person gets infected with two different versions of pathogen
  • Get chromosomal switches/mutations that result in a human virus with pig hemagglutinin
  • Then we don't have the neutralizing antibodies to get rid of it
• Antigenic Drift


  • Normally, neutralizing antibodies block binding site for hemagglutinin
  • But, virus replicates and produces mutation particularly in epitopes so neutralizing antibodies no longer work
• Programmed Cell rearrangement

  • Insect borne pathogens that live in extracellular spaces
  • Change the surface antigen repeatedly withing a single host
  • Immune system takes up so much energy trying to clear it
• Pathogens using Programmed Cell rearrangement

  • Sleeping sickness
  • Walking pneumonia
  • Malaria
• Latency


  • Some viruses go latent and thus make no viral proteins that are presented in MHC 1
  • They have long terminal repeats at the end of their genome that they insert into our DNA and then just hang out in our genome
  • Works well in things like neurons because they already carry low MHC 1 as they cannot easily be replaced, so immune system doesn't want to kill them off unless they absolutely have to
• Viruses that use Latency

  • Herpes/shingles
• At one time there usually only 1 strain of influenza that is causes cases all around the world
• Protective Immunity to influenza

  Developed through neutralizing antibodies to hemagglutinin (H1N1)
• Exploitation of immune response by viruses

  • Viruses can essentially steal our own genes and use them against us to help them evade immune response
  • Virally encode receptors to block antibodies
  • Virally encoded complement proteins to inhibit complement activation
• Immunosuppression

  • Superantigens: Cause many T cells to activate which produces large amounts of cytokines, causing toxic shock
  • T cells all undergo apoptosis leaving person immunosuppressed
  • Takes a while for new T cells to then be regenerated and there are not enough in the body to respond to pathogens
  • APC will still be working but they don't have anything to present to
• Inappropriate Immune Responses


  • Pathogen Induced: Leprosy example - Mycobacterium leprae hangs out in macrophage, needs TH1 to clear it
  • Human Induced: Vaccine Example (Bronchiolitis) - Infants were vaccinated with a vaccine that had alum added so that cells would clump up and be easier to clear by the immune system, but things clumped too much and didn't elicit neutralizing antibodies, instead children just had low dose chronic exposure causing a TH2 response and actually became much sicker