OCR A-Level Biology: Communicable Diseases and Immunity

A person is infected with a particular strain of bacterium for the first time. Several months later, after they have fully recovered, they are infected with the same strain again. On the second occasion they show no symptoms and clear the bacterium very quickly.

Describe and explain the differences between the primary and secondary immune responses to this bacterium. In your answer you must refer to the role of memory cells and explain why the secondary response is faster and larger than the primary response.

(6 marks)

Volunteers were given a vaccine against a bacterial antigen on day 0 and a booster dose of the same vaccine on day 28. The concentration of antibody specific to the antigen in their blood plasma was measured at intervals. The mean results are shown below.

Time / days	Mean antibody concentration / arbitrary units (a.u.)
0	0
7	2
14	8
28	6
31	60
35	144

(a) The peak concentration after the first dose was 8 a.u. (day 14) and the peak concentration after the booster was 144 a.u. (day 35). Calculate the fold-increase in peak antibody concentration produced by the booster compared with the first dose. Show your working. (2 marks)

(b) Explain why the booster dose produced a peak antibody concentration that was both higher and reached more quickly than the peak after the first dose. (4 marks)

A microbiologist investigated the sensitivity of a strain of bacterium to four antibiotics (P, Q, R and S) using a disc-diffusion assay. A lawn of the bacterium was spread over an agar plate using aseptic technique, paper discs each soaked in the same concentration of one antibiotic were placed on the surface, and the plate was incubated. After incubation the diameter of the clear zone of inhibition around each disc was measured. The results are the mean of three plates.

Antibiotic	Mean diameter of zone of inhibition / mm
P	22
Q	0
R	14
S	31

A separate control disc soaked only in sterile water produced a zone of inhibition of 0 mm.

(a) Explain what a clear zone of inhibition shows about the effect of an antibiotic on this bacterium, and explain why the bacterium is resistant to antibiotic Q. (3 marks)

(b) Using the data, state which antibiotic should be chosen to treat an infection with this strain, and explain the purpose of the water-only control disc. (2 marks)

A company has developed a lateral-flow test to detect a newly identified virus in a sample of a patient's saliva. The test uses two different monoclonal antibodies, both specific to a surface protein (antigen) of the virus:

• In the test zone, monoclonal antibody 1 is fixed to the strip and captures any virus present.
• A second monoclonal antibody, carrying a coloured dye, is free to move along the strip and also binds the captured virus, producing a coloured line only where virus has been trapped.

A coloured line appears in the test zone of a patient sample, but no line appears when the test is run on the saliva of a person known not to be infected.

Suggest and explain how the use of monoclonal antibodies allows this test to detect the virus specifically, and explain why no line forms for the uninfected person. (5 marks)

A new bacterial disease spreads through a region. A vaccine is developed and a national vaccination programme begins. Within two years the number of new cases each month falls almost to zero, including among the small number of people who were never vaccinated (for example newborn babies and people who could not be vaccinated for medical reasons).

Explain how vaccinating a large proportion of the population can protect the unvaccinated individuals from this disease. Use the term herd immunity in your answer. (4 marks)

Phagocytosis is part of the non-specific (innate) immune response. A phagocyte such as a neutrophil engulfs and destroys a bacterium.

Describe how a phagocyte engulfs and destroys a bacterium by phagocytosis. In your answer, name the organelle involved in destroying the bacterium and the substances it contains. (3 marks)