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Antibodies, vaccination and the four kinds of immunity questions

Antibody structure and how it fits function; agglutination, neutralisation and marking for phagocytosis; active and passive, natural and artificial immunity with examples; vaccination, herd immunity and the threshold; antigenic variation and the annual influenza vaccine; monoclonal antibodies; and the ethical questions raised by vaccine trials.

7 original questions · 24 marks · the antibodies, vaccination and the four kinds of immunity notes · Pathogens, disease and immunity

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• 4 marks · Explain

  An antibody has no enzymic activity of any kind. Explain how antibodies nevertheless help to clear a bacterial infection.

  Mark scheme

  1. B1 an antibody has two binding sites and cannot lyse a cell, digest a wall or poison anything; all it does is bind an antigen to form an antigen-antibody complex
  2. B1 agglutination: because each antibody has two identical binding sites it can bind two bacteria at once, clumping them so they cannot spread and so one phagocyte can engulf many at a time
  3. B1 neutralisation: antibody bound over a toxin, or over the attachment proteins of a pathogen, covers the part that would otherwise fit a host receptor, so it cannot act or enter a cell
  4. B1 marking for phagocytosis: the constant regions project outwards and phagocytes carry receptors complementary to them, so the pathogen is engulfed and hydrolysed inside the phagocyte

• 4 marks · Compare

  Compare active immunity with passive immunity, referring to how each arises, how quickly it acts and how long it lasts.

  Mark scheme

  1. B1 in active immunity the person's own lymphocytes meet the antigen and respond, whereas in passive immunity ready-made antibody is introduced from outside
  2. B1 active immunity takes days or weeks to become established, whereas passive immunity works immediately because the antibody is already present
  3. B1 active immunity leaves memory cells behind, whereas passive immunity produces none
  4. B1 active immunity therefore lasts for years, whereas passive immunity fails within weeks as the introduced antibodies are broken down and nothing replaces them

• 4 marks · Suggest

  An ELISA test for HIV detects antibodies against the virus rather than the virus itself. Suggest why a person infected two weeks ago may still test negative, and suggest what follows for the screening of donated blood.

  Mark scheme

  1. B1 after infection the specific response takes time: antigen must be presented, a B cell selected, and clonal expansion must run before plasma cells exist
  2. B1 a primary response takes about eight days to produce detectable antibody and the concentration rises slowly, so it may still be below what the test can detect
  3. B1 the person is infected, and infectious, despite the negative result, which is a false negative
  4. B1 donated blood could therefore transmit HIV despite passing the screen, so an antibody test alone is not sufficient and a further test or a repeat test after an interval is needed

• 4 marks · Evaluate

  A new vaccine is trialled during an epidemic, with one group given the vaccine and a control group given a placebo. Evaluate the use of a placebo control group in a trial of this kind.

• 3 marks · Calculate

  A virus has a basic reproduction number R₀ of 12 in a fully susceptible population. Calculate the percentage of that population which must be immune before transmission is no longer sustained.

• 3 marks · Explain

  Explain why the influenza vaccine has to be reformulated every year while the measles vaccine does not.

• 2 marks · Identify

  A baby receives antibodies in its mother's breast milk. A child of five is given the MMR vaccine. Identify the type of immunity produced in each case, naming both axes of the classification.

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