Germ strategies for evading immunity

• grow fast

  • influenza

• mutate antigens

  • influenza (RNA virus; RNA-copying enzymes are much less accurate than DNA-copying enzymes)

  • Streptococcus

  • AIDS (caused by an RNA virus)

  • trypanosomes

S. pneumoniae capsular antigens

• the various types of S. pneumoniae differ in their capsular polysaccharides

• an IgM response that clears one infection probably will not respond at all to a subsequent infection of a different type

antigenic drift and antigenic shift

• antigenic drift: if a microbial antigen can mutate rather quickly, then these mutations can evade antibodies

• antigenic shift: hybrid viruses can evade antibody recognition

trypanosome VSG genes

• this looks like our V region genes coding for antibodies

• throughout evolution, as we shifted our V region genes around, they shifted their capsular genes around

• so this response worked in both the organism and the host

• the difference is that the VSG aren’t joined in a way such that there is a mutation every time you join segments; antibodies are special because of sloppy joining between V, D, and J regions, and these sloppy areas are places of antigen recognition. furthermore, antibodies involve somatic mutation

• any trypanosome can go through many cycles of gene conversion

• a person with infection by trypanosomes will manifest surges of different trypanosome types, each followed by an immune response which is then followed by another trypanosome type. this can take a very long time to clear

Category: Pathology Notes