Vaccines train the immune system to recognize a dangerous pathogen before a person is exposed to the real infection. They do this by safely presenting antigens, which are molecular clues from a virus or bacterium, without usually causing the disease. This matters because the immune system can respond faster and more strongly after training.
Vaccination protects individuals and can also protect whole communities when enough people are immune.
After vaccination, antigen-presenting cells display vaccine antigens to helper T cells, which help activate B cells and other immune cells. Some B cells become plasma cells that make antibodies, while others become memory B cells that can last for years. If the real pathogen appears later, memory cells rapidly produce a stronger secondary immune response.
Different vaccine types, including live attenuated, inactivated, and mRNA vaccines, use different methods to deliver antigen instructions or antigen material.
Key Facts
- Antigens are molecules that immune cells recognize as specific signals from a pathogen or vaccine.
- Primary immune response: first exposure leads to slower antibody production and formation of memory cells.
- Secondary immune response: later exposure triggers faster, stronger antibody production by memory B cells.
- mRNA vaccines deliver genetic instructions so body cells make a harmless antigen, usually a viral protein.
- Herd immunity threshold is approximately H = 1 - 1/R0, where R0 is the basic reproduction number.
- Vaccine effectiveness can be estimated as VE = (risk in unvaccinated - risk in vaccinated) / risk in unvaccinated x 100%.
Vocabulary
- Vaccine
- A vaccine is a preparation that safely trains the immune system to recognize and respond to a pathogen.
- Antigen
- An antigen is a molecule, often a protein or sugar, that immune cells can recognize as a target.
- Antibody
- An antibody is a Y-shaped protein made by B cells that binds specifically to an antigen.
- Memory cell
- A memory cell is a long-lived immune cell that responds quickly if the same antigen appears again.
- Herd immunity
- Herd immunity occurs when enough people in a population are immune that a pathogen has difficulty spreading.
Common Mistakes to Avoid
- Thinking vaccines work immediately is wrong because the immune system usually needs days to weeks to build strong antibody and memory cell responses.
- Confusing antigens with antibodies is wrong because antigens are the targets being recognized, while antibodies are immune proteins that bind to those targets.
- Assuming mRNA vaccines change DNA is wrong because mRNA is read in the cell cytoplasm and does not become part of the cell's genome.
- Believing herd immunity means everyone is protected equally is wrong because people with weak immune systems or no vaccine response can still be at risk.
Practice Questions
- 1 A disease has R0 = 5. Using H = 1 - 1/R0, what fraction and percentage of the population would need immunity to reach the herd immunity threshold?
- 2 In a study of 10,000 unvaccinated people, 500 become infected. In a similar group of 10,000 vaccinated people, 50 become infected. Use VE = (risk in unvaccinated - risk in vaccinated) / risk in unvaccinated x 100% to calculate vaccine effectiveness.
- 3 Explain why a vaccinated person can produce antibodies faster after exposure to the real pathogen than an unvaccinated person encountering it for the first time.