C3.2 Defence against disease. Practice questions with markscheme.

• C: skin and mucous membranes block entry; antibodies belong to the adaptive response after infection;

• C: pathogens are diverse — bacteria (e.g. tuberculosis), viruses (e.g. influenza), fungi (e.g. ringworm) and protists (e.g. malaria) all include human pathogens; blood cells, lymphocytes and antibodies are the body's own components;

• independent variable: the antibiotic on the disc — three named antibiotics, each at the same concentration;
• dependent variable: diameter (or area) of the clear zone of inhibition around each disc, measured after a fixed incubation period;
• controlled variables: concentration and volume of antibiotic per disc, disc diameter, age and concentration of the bacterial culture spread on the plate, depth and composition of the agar, incubation temperature and time;
• replication: each antibiotic tested on at least three separate plates, with a mean zone diameter calculated;
• a disc soaked in sterile water or in the solvent alone is included as a negative control;
• aseptic technique is used throughout, e.g. flaming the spreader and sealing the plates before incubation;

Aseptic and safety points alone cannot gain more than one mark. Award marks only where a variable is clearly identified as independent, dependent or controlled.

• HIV is transmitted in body fluids that contain the virus / infected lymphocytes, e.g. blood, semen, vaginal secretions, breast milk;
• through unprotected sexual intercourse (vaginal or anal) with an infected person;
• through blood, e.g. sharing of hypodermic needles by intravenous drug users or transfusion of unscreened blood / blood products;
• from mother to child across the placenta, during childbirth or in breast milk;
• HIV is not transmitted by casual contact, insect bites, or sharing food, because the virus does not survive long outside the body, OWTTE;

Award [1] for body fluids with at least two named examples and [1] for each further correct point (a route of transmission or the point about casual contact), to a maximum of [3].

• axes labelled: time / days on the horizontal axis and concentration of specific antibody (arbitrary units) on the vertical axis, with both injections marked on the time axis;
• after the first injection there is a lag of several days before antibody appears;
• the primary response is drawn as a low peak that then falls;
• after the second injection antibody appears sooner and rises more steeply than in the primary response;
• the secondary peak is much higher than the primary peak and falls more slowly / remains raised;

No numerical values are required; credit shape and trend only. Both responses must be distinguishable.

• (a) [2]
  • as coverage rises, cases fall (strong negative correlation);
  • the fall is steepest at high coverage / cases approach zero as coverage passes ~90 %, OWTTE;
• (b) [1]
  • 99.97 % (accept ~99.9–100 %); (480 000 − 140)/480 000 × 100;
• (c) [3]
  • measles is highly contagious, so herd immunity needs very high coverage (~95 %);
  • a fall in coverage leaves (clusters of) susceptible people, especially if unvaccinated individuals are concentrated in communities;
  • (so) transmission chains can re-establish and outbreaks occur (imported cases spread), OWTTE;
• (d) [2]
  • other factors changed over the period (nutrition, healthcare, surveillance), correlation is not causation;
  • supporting evidence: mechanism (immune memory), controlled trials, outbreaks tracking unvaccinated groups, rebound when coverage falls, OWTTE;