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3.7.11Inquiry Process

Vaccines and Immunity — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 152 of 160

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Part 1 of 5How Vaccines Work

3.7.11 Vaccines and Immunity
1
A vaccine introduces antigens from a pathogen, in a weakened, inactivated or partial form, or as instructions to make an antigen.
2
B lymphocytes with matching receptors are activated, divide, and produce plasma cells that secrete antibodies.
3
Some activated lymphocytes become memory cells, which remain in the body for years.
4
On exposure to the real pathogen, memory cells respond more quickly, producing more antibodies in a secondary response.
5
The pathogen is destroyed before it can cause illness, so the person is immune without having had the disease.

Part 2 of 5Active and Passive Immunity

Active immunity
  • The body makes its own antibodies and memory cells, either after infection or after vaccination, giving long-lasting protection.
Passive immunity
  • Antibodies are acquired from another source,
    • such as across the placenta or in breast milk,
    • giving protection that is rapid but temporary.
Memory cells
  • Present only in active immunity, they account for the faster and stronger secondary response.
Boosters
  • Repeated doses strengthen memory for pathogens whose antigens change or whose protection fades over time.
FeatureActivePassive
Antibodies made byOwn plasma cellsAnother individual
Memory cellsYesNo
OnsetSlowImmediate
DurationLongShort

Part 3 of 5Herd Immunity

Herd Immunity
Concept
  • If enough of a population is immune, an infected person has too few susceptible contacts for the disease to spread widely.
Protection of others
  • People who cannot be vaccinated, such as infants or those with weak immunity, are protected indirectly.
Threshold
  • The proportion that must be immune is about 1 − 1/R₀,
    • where R₀ is the average number of people one case infects in a fully susceptible population.
Falling coverage
  • If vaccination rates fall below the threshold, outbreaks of diseases such as measles can return.
Eradication
  • Smallpox was eradicated because high vaccination coverage cut off its transmission, showing the power of herd immunity.

Part 4 of 5Worked Example

Question

  • Measles has an R₀ of about 15.
  • Calculate the approximate proportion of the population that must be immune for herd immunity.

  1. Threshold = 1 − 1/R₀.
  2. Substitute R₀ = 15: 1 − 1/15 = 1 − 0.0667.
  3. Threshold = 0.933, that is 93.3%.

Answer About 93% of the population must be immune.

Part 5 of 5Exam Checklist & Tips

3.7.11 – Key Points for the Exam

  • Vaccine A preparation of antigens that stimulates an immune response without causing disease.
  • Memory cell Long-lived lymphocyte that allows a rapid response to a repeat infection.
  • Herd immunity Indirect protection of a population when a large proportion is immune.
  • R₀ Average number of new infections caused by one case in a fully susceptible population.

Common Mistakes

  • Saying that vaccines contain antibodies; they contain antigens (active immunity).
  • Believing herd immunity requires 100% vaccination.
  • Confusing passive immunity, which has no memory cells, with active immunity.

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