vaccines and herd immunity

• Passive immunity


  When you don't actually create the antibodies yourself, but have the antibodies introduced into you
• Examples of passive immunity


  • Antibodies passed through the placenta to a fetus from the mother
  • Antibodies passed through breast milk to a baby
  • Injecting antibodies to neutralize snake venom
• Active immunity


  When you have created the antibodies yourself, after being exposed to the pathogen or antigen
• Types of active immunity

  • Natural active immunity (from natural infection)
  • Artificial active immunity (from vaccines)
• Vaccine


  A weak, weakened, or dead form of a pathogen, or its antigens, that is introduced to activate the immune system
• Immune response to a vaccine

  1. Lymphocytes exposed to antigen

  2. B-cells undergo clonal expansion and differentiation

  3. Plasma cells produce antibodies

  4. Memory B-cells remain in blood for decades

  5. Upon re-exposure, memory B-cells rapidly divide and produce large quantities of antibodies
• Primary vs secondary immune response

  Primary response is to initial vaccine exposure, secondary response is to natural infection after vaccination
• Herd immunity


  When a large enough proportion of a population is vaccinated, it protects those who cannot be vaccinated
• It is impossible to vaccinate every single person in a large population
• Reasons some cannot be vaccinated include young age, compromised immune systems, or other illnesses
• Without herd immunity


  Pathogens can easily spread among unvaccinated individuals
• With herd immunity


  Pathogens have difficulty spreading, even if some individuals are unvaccinated
• Antigen variability can compromise vaccine effectiveness over time, requiring updates (e.g. for influenza)