All revision notes topics

Immunity and the evolutionary raceEdexcel International A Level Biology: Revision notes

Section 1

Active and passive immunity

Active immunity is when the body makes its own antibodies, after being exposed to an antigen. It produces memory cells, so protection is long-lasting, but it takes time to develop.

Passive immunity is when antibodies are given to the body from another source. It acts immediately but is short-lived, because the antibodies are proteins that are broken down and no memory cells are made.

Key termsactive immunitypassive immunity

Section 2

Natural and artificial immunity

Each type of immunity can be natural (without medical intervention) or artificial (as a result of medical treatment).

  • Natural active – the body makes antibodies after catching an infection
  • Artificial active – a vaccination introduces antigens so that the body makes antibodies and memory cells
  • Natural passive – antibodies pass from mother to baby across the placenta or in breast milk
  • Artificial passive – ready-made antibodies are injected, e.g. against rabies or tetanus
Key termsnatural immunityartificial immunityvaccination
Exam tip

Sort every example with two questions: does the body make the antibodies (active or passive), and was medicine involved (artificial or natural)?

Section 3

Pathogen evasion mechanisms

Pathogens have evolved ways to avoid the immune system:

  • Antigenic variation – mutation changes surface antigens (e.g. influenza, HIV), so existing memory cells and antibodies no longer recognise them
  • Capsules – a polysaccharide capsule (e.g. Streptococcus pneumoniae) hides antigens and prevents phagocytosis
  • Survival inside host cells – Mycobacterium tuberculosis stops lysosomes fusing with the phagosome in macrophages, so it survives hidden from antibodies
  • Attacking the immune system – HIV destroys T helper cells, weakening the whole response
Key termsantigenic variationcapsule

Section 4

The evolutionary race

Pathogens and hosts exert selection pressure on each other:

  1. Random mutation gives pathogen variants.
  2. A variant that evades the immune system has a selective advantage, survives and reproduces, and passes on the allele.
  3. The allele frequency increases in the pathogen population.
  4. Hosts with variant immune genes that overcome the evasion are more likely to survive and reproduce, so host defences evolve.
  5. This puts a new selection pressure on the pathogen.

The result is a continuing evolutionary race. It explains why flu vaccines must be updated every year.

Key termsevolutionary raceselection pressure
Common mistake

Pathogens do not mutate 'in order to' evade immunity. Mutation is random, and selection favours the variants that happen to evade it.

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Exam questions on Immunity and the evolutionary race

  1. A baby is breast-fed and receives antibodies from her mother in the milk. At eight weeks old she is also given her first dose of a vaccine against whooping cough.
    Distinguish between active and passive immunity.2 marks
  2. A traveller is bitten by a dog in a region where rabies occurs. At the clinic he is immediately injected with ready-made rabies antibodies, and is then given a course of rabies vaccine over the following weeks.
    Explain why the injected antibodies give the traveller only short-term protection.2 marks
  3. Influenza viruses accumulate mutations in the genes for their surface antigens, so a new strain appears every few years. Each year a new influenza vaccine has to be produced against the strains most likely to be circulating.
    Explain why a person who has recovered from influenza one year may become ill again when a new strain appears.3 marks
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).