Lecture Notes on Immunity and Vaccination

Herd Immunity and Pathogen Mutation

• Herd immunity relies on close exposure and genetic/background factors for natural infection.
• If a pathogen mutates, herd immunity may no longer be effective.
• The level of natural immunity can vary among individuals.

Antibody Titer in Serum: Principle of Vaccine

• Antibody titer in serum is a way to measure the immune response, which the vaccine takes advantage of.

Primary Response

• Upon the initial encounter with an antigen:
  • Activated B cells, called plasma cells, produce IgM specific to that antigen.
  • Subsequently, IgG specific to the antigen is also produced by the same plasma cells through class switching.

Secondary Response

• Upon the second encounter with the same antigen:
  • Antibody titer increases more drastically.
  • The peak response is reached in a shorter amount of time (between 2-7 days).
• Secondary response is mediated by memory B cells:
  • A subset of activated B-cells that did not become plasma cells.
  • These cells are long-lived (up to decades).
  • Memory B cells undergo rapid clonal expansion upon encountering the same antigen.
  • This produces more rapid and stronger protection, which is the basis for vaccine effectiveness.

Antibody timeline

*   Initial exposure to antigen.
*   IgM production begins around day 7, followed by IgG production.
*   Second exposure to antigen on day 52.
*   Rapid increase in IgG production, while IgM levels remain relatively low. Peak response between 58-65 days.