CIE A Level Biology复习笔记11.2.5 How Vaccines work

• A vaccine is a suspension of antigens that are intentionally put into the body to induce artificial active immunity. A specific immune response where antibodies are released by plasma cells
• There are two main types of vaccines:
  • Live attenuated
  • Inactivated
• Vaccines are administered either by injection or orally (by mouth). When a person is given a vaccine they have been given a vaccination
• The vaccinations given by injection can be into a vein or muscle
• Vaccinations produce long-term immunity as they cause memory cells to be created. The immune system remembers the antigen when reencountered and produces antibodies to it, in what is a faster, stronger secondary response

• Vaccines can be:
  • Highly effective with one vaccination giving a lifetime’s protection (although less effective ones will require booster / subsequent injections)
  • Generally harmless as they do not cause the disease they protect against because the pathogen is killed by the primary immune response
• Unfortunately there can be problems with vaccines:
  • People can have a poor response (eg. they are malnourished and cannot produce the antibodies – proteins or their immune system may be defective)
  • A live pathogen may be transmitted (e.g. through faeces) to others in the population (ideally enough number of people are vaccinated at the same time to give herd immunity)
  • Antigenic variation – the variation (due to major changes) in the antigens of pathogens causes the vaccines to not trigger an immune response or diseases caused by eukaryotes (eg. malaria) have too many antigens on their cell surface membranes making it difficult to produce vaccines that would prompt the immune system quickly enough
  • Antigenic concealment – this occurs when the pathogen ‘hides’ from the immune system by living inside cells or when the pathogen coats their bodies in host proteins or by parasitising immune cells such as macrophages and T cells (eg. HIV) or by remaining in parts of the body that are difficult for vaccines to reach (eg. Vibrio cholerae – cholera, remains in the small intestine)
• The principles underpinning vaccinations were discovered by Edward Jenner in the 1700s when he developed the first smallpox vaccine

Live attenuated vaccines

• Live attenuated vaccines contain whole pathogens (e.g. bacteria and viruses) that have been ‘weakened’
• These weakened pathogens multiply slowly allowing for the body to recognise the antigens and trigger the primary immune response (plasma cells to produce antibodies)
• These vaccines tend to produce a stronger and longer-lasting immune response
• They can be unsuitable for people with weak immune systems as the pathogen may divide before sufficient antibodies can be produced
• An example of this type of vaccine is the MMR (Measles, Mumps and Rubella)

Inactivated vaccines

• Inactivated vaccines contain whole pathogens that have been killed (‘whole killed’) or small parts (‘subunit’) of the pathogens (eg. proteins or sugars or harmless forms of the toxins – toxoids)
• As inactivated vaccines do not contain living pathogens they cannot cause disease, even for those with weak immune systems
• However these vaccines do not trigger a strong or long-lasting immune response like the live attenuated vaccines. Repeated doses and / or booster doses are often required
• Some people may have allergic reactions or local reactions (eg. sore arm) to inactivated vaccines as adjuvants (eg. aluminium salts) may be conjugated (joined) to the subunit of the pathogen to strengthen and lengthen the immune response
• An example of a whole killed vaccine is polio vaccine
• An example of a toxoid subunit vaccine (where inactivated versions of the toxins produced by pathogens are used) is Diphtheria