Immunisation

• Vaccines save up to 3 million lives per year according to the WHO
• Definitions:

  • Antigen: A live or inactivated substance capable of producing an immune response
  • Antibody: Protein molecules produced by B lymphocytes to help eliminate an antigen
  • Adjuvant: Substances that enhance the antibody response, often found in combination vaccines
• Principles of Immunisation:

  • Immunisation is the process of inducing or providing immunity to an infectious disease artificially
  • Passive immunisation involves transferring pre-formed antibodies for immediate but short-term protection
  • Active immunisation stimulates the host to produce an immune response, usually long-lasting
• Active immunity:

  • Stimulates the host to produce a primary immune response
  • Results in a secondary response upon re-exposure to the pathogen, ideally providing lifelong protection
• Live vaccines:

  • Attenuated (weakened) agents that mimic natural infection responses
  • Examples include BCG, rubella, measles, and polio
• Non-Live vaccines:

  • Cannot replicate and do not cause disease
  • Examples include inactivated polio, hepatitis A, and tetanus
• Live attenuated vaccine example: Yellow fever
• Non-live vaccines do not replicate and do not cause the disease they are trying to prevent
• The more similar a vaccine is to the disease-causing form of the organism, the better the immune response to the vaccine
• Types of non-live vaccines:

  • Whole inactivated
  • Toxoid (inactivated toxin)
  • Fractional/subunit
  • Protein-based
  • Polysaccharide-based
  • Nucleic acid-based
• Whole inactivated vaccines:

  • Whole pathogen killed (by heat or formalin) and injected to induce an antibody response
  • Not as effective as live vaccines
  • Require 3-5 doses
  • The immune response produced is mostly humoral (antibody)
  • Antibody titre falls over time
  • Examples: Inactivated polio vaccine, Hepatitis A vaccine
• Toxoid vaccines:

  • Modified bacterial toxin that has been rendered non-toxic but still remains antigenic
  • Stimulates the formation of antibody to the toxin
  • Creates immunity to the toxin that causes disease, not the bacteria
  • Examples: Diptheria, Tetanus
• Protein-based vaccines:

  • Contains only specific antigenic proteins of an infectious agent
  • Example: acellular pertussis – pertussis toxoid + proteins from the surface of the pertussis bacteria
• Polysaccharide-based vaccines:

  • Contains polysaccharide from bacteria surface capsule
  • Not consistently immunogenic in children <2 years of age
  • No booster response
  • Produce antibody with less functional activity than that produced by the infection
  • Example: Adult pneumococcal vaccine
• Fractional/subunit vaccines:

  • Proteins are more immunogenic than polysaccharides
• Conjugate Polysaccharide vaccines:

  • Polysaccharide chemically linked to a protein
  • Induces a more potent immune response
  • Increased immunogenicity in infants
  • Antibody booster response to multiple doses of vaccine
  • Examples: Hib vaccine, MenC vaccine, PCV (children’s pneumococcal vaccine), MenACWY
• In multicellular organisms, the distance for substances to enter cells is larger due to a higher surface area to volume ratio
• Multicellular organisms require specialised exchange surfaces for efficient gas exchange of carbon dioxide and oxygen