Topic 10: Diseases and Immunity -- Challenge Exam 2
1 hour 15 minutes
80
7
75:00
0610
Instructions
Answer all questions in the spaces provided.
Show all working for calculations.
Use appropriate scientific terminology.
Your answers will be automatically graded when you submit.
Question Navigation
This paper covers the whole topic. Like a real Cambridge paper, the seven questions range across every sub-topic of Topic 10 — pathogens and transmission, body defences and control, active immunity and vaccination, passive immunity, and cholera. Cambridge itself numbers all of this as a single section, 10.1; the 10.1–10.5 split used on this site is ours, for teaching.
Question 1 — Barriers, Cells and the Routes They Guard
Total: 12 marks
(a)[3]
The syllabus groups the body defences by the way they work. State one physical barrier, one chemical barrier and one cellular defence, and describe how each works.
Model Answer — 1a
physical: skin (a continuous barrier while unbroken), hairs in the nose (trap dust and particles) or mucus (traps pathogens, swept away by cilia) [1]
chemical: stomach acid — kills most pathogens taken in with food and drink [1]
cellular: white blood cells — phagocytes engulf and digest pathogens, lymphocytes produce antibodies [1]
⚠ If you missed marks here: Each mark needs the mechanism as well as the name. Note also that stomach acid is the only chemical barrier in the syllabus list, so an answer offering “sweat” or “tears” there scores nothing.
(b)[3]
A hospital reduces infections by insisting that staff wash their hands between patients, and by covering all wounds with dressings. Explain, in terms of transmission routes and body defences, how each of these measures works.
Model Answer — 1b
hands act as a contaminated surface, carrying pathogens from one patient to another — an indirect route [1]
washing removes the pathogens before they can be transferred, so the route is closed [1]
a wound is a break in the skin, the body’s physical barrier, so covering it restores a barrier against pathogens entering the blood [1]
⚠ If you missed marks here: Two measures, two mechanisms, and they are not the same mechanism. Hand washing closes a transmission route outside the body; a dressing replaces a body defence that has been broken.
(c)[4]
A gut disease in a town is spread by drinking water contaminated with sewage, by food prepared by people who have not washed their hands, and by flies that visit uncovered rubbish. For each of these three routes, name one control measure, and then state which single measure would help with more than one route.
Model Answer — 1c
water route: a clean water supply, or treating the water so it contains no pathogens [1]
food route: hygienic food preparation and good personal hygiene, especially hand washing [1]
flies: waste disposal, removing the rubbish that flies breed on and feed on [1]
sewage treatment helps with more than one route, because it removes the bacteria from human waste before they can reach the water, the food or the flies [1]
⚠ If you missed marks here: The last mark rewards you for seeing that all three routes lead back to the same source, human faeces. That is also why sewage treatment and a clean water supply are listed separately in the syllabus.
(d)[2]
A virus destroys many of the ciliated cells lining the trachea. Suggest why patients often develop a second, bacterial infection of the lungs afterwards.
Model Answer — 1d
mucus traps pathogens but is normally swept away by the ciliated cells [1]
with the cilia destroyed the loaded mucus is not removed, so trapped bacteria remain in the airway and can reach the lungs [1]
⚠ If you missed marks here: Mucus and cilia are a partnership: one traps, the other removes. Losing either half leaves the pathogens sitting where they were caught, which is why this defence is described as trapping and sweeping away.
Question 2 — A National Vaccination Programme
Total: 12 marks
The table shows the percentage of children vaccinated against a virus in one country, and the number of cases of the disease reported each year.
Year
1
5
10
15
18
21
Children vaccinated / %
12
58
91
94
71
63
Cases reported
18 400
4 100
210
36
1 950
5 600
(a)[2]
Calculate the percentage decrease in reported cases between year 1 and year 15. Show your working.
Model Answer — 2a
18 400 − 36 = 18 364 [1]
18 364 ÷ 18 400 × 100 = 99.8 % [1]
⚠ If you missed marks here: Percentage change is always change divided by the original, and the original here is the year 1 figure. Dividing by 36 instead would give an answer of over 50 000 %, which a quick sanity check catches at once.
(b)[3]
Explain how vaccinating a high proportion of children reduced the number of cases to almost zero by year 15.
Model Answer — 2b
vaccinated children produced antibodies and memory cells, so they did not develop the disease [1]
so the pathogen did not multiply in them and they could not pass it on [1]
with over 90 % of children unable to transmit it, the pathogen could not spread and transmission almost stopped [1]
⚠ If you missed marks here: The first mark is about individuals and the last two are about transmission. A question that gives you a whole population is asking you to move from one to the other, and answers that stop at “they were immune” score one mark of three.
(c)[3]
Explain the pattern seen after year 15, and suggest which children would be at greatest risk.
Model Answer — 2c
vaccination fell from 94 % to 63 %, so a much larger number of children had no immunity [1]
the pathogen then had enough hosts to move between, so transmission was re-established and cases rose to 5600 [1]
those at greatest risk are the unvaccinated children, and babies too young to be vaccinated whose passive immunity from the mother has faded [1]
⚠ If you missed marks here: Use the figures: quoting the fall from 94 to 63 % alongside the rise in cases is usually a mark of its own. The babies matter because passive immunity leaves no memory cells, so once it fades they have nothing.
(d)[4]
Outline the process by which a vaccine produces long-term immunity in a child.
Model Answer — 2d
weakened pathogens or their antigens are put into the body [1]
the antigens stimulate an immune response by lymphocytes [1]
the lymphocytes produce antibodies, which have a shape complementary to the antigen [1]
memory cells are produced and remain, so a later infection produces antibodies faster and in greater quantity [1]
⚠ If you missed marks here: Four marks means four steps. The memory cell step is the one that answers the phrase long-term in the question, so an answer ending at antibodies cannot score full marks however well written it is.
Question 3 — Testing Whether an Antibody Fits
Total: 12 marks
Antibodies were collected from four people, each immune to a different disease. Each sample was mixed with bacterium X, and the percentage of bacteria clumped together after 10 minutes was recorded.
Antibody sample
from a person immune to
Bacteria clumped / %
1
the disease caused by bacterium X
94
2
cholera
3
3
measles
2
4
influenza
4
(a)[3]
Explain what is meant by an antigen and by an antibody, stating clearly where each is found and what produces it.
Model Answer — 3a
an antigen is a molecule found on the surface of a pathogen [1]
an antibody is a protein [1]
made by a lymphocyte, a white blood cell, inside the body [1]
⚠ If you missed marks here: Where is it, and who made it. Those two questions take five seconds and prevent the error that costs more marks in Topic 10 than anything else — using the two words the wrong way round.
(b)[3]
Explain the results in the table.
Model Answer — 3b
each pathogen has its own antigens, which have specific shapes [1]
only the antibodies from sample 1 have a shape complementary to the antigens on bacterium X, so they bind and clump 94 % of the bacteria [1]
the antibodies in samples 2, 3 and 4 are complementary to different antigens, so they cannot bind and only 2–4 % clumping occurs, which is close to background [1]
⚠ If you missed marks here: Use the figures rather than saying “the others did not work”. Note too that the samples all contain perfectly functional antibodies — the reason they fail is shape, not strength or age.
(c)[4]
Describe how the body of a person infected with bacterium X for the first time would respond, naming the cells involved and the two possible fates of the bacteria.
Model Answer — 3c
phagocytes engulf and digest bacteria by phagocytosis, without needing to have met them before [1]
the antigens on bacterium X stimulate lymphocytes [1]
the lymphocytes produce antibodies with a complementary shape, which bind to the antigens [1]
the bacteria are then destroyed directly, or marked for destruction by phagocytes [1]
⚠ If you missed marks here: Two cells, two jobs, and the mark scheme wants both. Phagocyte: swallow. Lymphocyte: label. An answer in which the lymphocyte engulfs anything has swapped them.
(d)[2]
The person recovers. Explain why they are unlikely to become ill from bacterium X again, but could still catch cholera.
Model Answer — 3d
memory cells for the antigens of bacterium X remain and produce antibodies rapidly and in large quantity if it returns, so the bacteria are destroyed before symptoms appear [1]
the cholera bacterium has different antigens, so those memory cells and antibodies are not complementary to it and give no protection [1]
⚠ If you missed marks here: The same fact — specificity — explains both halves. If the second half of your answer would still work with the word “tired” or “run down” in it, it is not a biology answer.
Question 4 — A Cholera Outbreak in a River Town
Total: 12 marks
A town of 9000 people had 261 cases of severe watery diarrhoea in one week. The sewage outflow enters the river 300 m upstream of the point where water is collected.
Water source used by household
Cases per 1000 people
river
44
covered but untreated village well
17
treated piped water
2
(a)[2]
Calculate how many times more likely a household using river water was to have a case than a household using treated piped water, and state what the data suggest about the transmission route.
Model Answer — 4a
44 ÷ 2 = 22 times [1]
the disease is transmitted in contaminated water, since the risk depends on the water source used [1]
⚠ If you missed marks here: Check the direction before writing: the river group has the higher rate, so the answer must be greater than one. Ratios inverted by accident are among the commonest single-mark losses in data questions.
(b)[3]
Explain the figure of 17 cases per 1000 for households using the covered well.
Model Answer — 4b
the well is covered, so it does not receive sewage directly from the river and the rate is far below the river figure of 44 [1]
but it is untreated, so it can still be contaminated by seepage, by dirty containers or by unwashed hands [1]
so the rate is well above the treated piped supply at 2 per 1000 [1]
⚠ If you missed marks here: The middle value is where the marks hide. A good answer explains why it is lower than one figure and higher than the other; an answer that only says “the well is dirty” explains neither comparison.
(c)[4]
Explain how the pathogen responsible causes the severe watery diarrhoea seen in these patients.
Model Answer — 4c
the bacterium multiplies in the small intestine and produces a toxin [1]
the toxin causes the cells lining the small intestine to secrete chloride ions into the gut [1]
this gives the gut contents a lower water potential than the blood and cells [1]
so water moves into the gut by osmosis through partially permeable membranes, and the faeces are watery [1]
⚠ If you missed marks here: Four links and none may be skipped. The chloride ion step is the one most often left out, and without it there is nothing to explain why the water moves — a toxin cannot pull water across a membrane.
(d)[3]
A councillor proposes spending the whole health budget on vaccinating the town. Evaluate this proposal.
Model Answer — 4d
in favour: vaccination would give active immunity with memory cells to those who receive it, reducing the number of people who become ill [1]
against: it does not close the transmission route — the sewage still enters the river above the collection point, so everyone missed by the programme is exposed repeatedly [1]
overall: treating the sewage and providing clean water would protect the whole town at once and also prevent other water-borne diseases, so it should come first [1]
⚠ If you missed marks here: Evaluate means giving the proposal its due before rejecting it. An answer that only disagrees is half an answer, and one that only agrees ignores the strongest evidence in the question — the position of the sewage outflow.
Question 5 — Borrowed Antibodies
Total: 10 marks
(a)[2]
Define passive immunity.
Model Answer — 5a
a short-term defence against a pathogen [1]
by antibodies acquired from another individual [1]
⚠ If you missed marks here: Two clauses, two marks, and the word short-term is one of them. The definition is deliberately built from the same words as active immunity, so leaving out either clause makes it impossible to tell which one you mean.
(b)[3]
Explain the importance of breastfeeding in the development of passive immunity in an infant.
Model Answer — 5b
breast milk contains antibodies made by the mother, which pass to the infant [1]
the infant’s own immune system is not yet fully developed and it has met few antigens, so it produces very few antibodies of its own [1]
the mother’s antibodies therefore give immediate protection during the months when the infant is least able to defend itself [1]
⚠ If you missed marks here: The antibodies come ready-made, which is why protection is immediate. An answer saying that milk contains antigens has turned this into active immunity and cannot then explain why the protection fades.
(c)[3]
Compare active and passive immunity. Make three comparisons.
Model Answer — 5c
in active immunity the antibodies are produced by the person’s own lymphocytes, whereas in passive immunity they come from another individual [1]
active immunity develops slowly, over days or weeks, whereas passive immunity is immediate [1]
active immunity produces memory cells and is long-term, whereas passive immunity produces no memory cells and is short-term [1]
⚠ If you missed marks here: A comparison must mention both things in the same statement, or the examiner cannot tell which one you mean. Two true sentences written separately often score one mark between them where a single linked sentence would score two.
(d)[2]
Explain why no memory cells are produced in passive immunity.
Model Answer — 5d
memory cells are produced by lymphocytes that have been stimulated by an antigen [1]
in passive immunity only finished antibodies enter the body; no antigen enters, so the lymphocytes are never stimulated [1]
⚠ If you missed marks here: This follows from the definition, so it never needs learning as a separate fact. It is also the reason the childhood vaccination schedule exists at all: once the borrowed antibodies are broken down there is nothing left behind.
Question 6 — Designing a Test of a New Vaccine
Total: 12 marks
A vaccine against a bacterial disease was tested on four groups of 200 mice. All mice were the same age, sex and mass and were kept in identical conditions.
Group
Treatment
Then exposed to the bacterium?
Number becoming ill
1
vaccine
yes
7
2
sterile salt solution
yes
168
3
vaccine
no
0
4
antibodies from a recovered mouse
yes, same day
12
(a)[3]
Explain the purpose of group 2 and of group 3.
Model Answer — 6a
group 2 shows how many mice become ill without the vaccine, giving a baseline for comparison — 7 out of 200 means nothing without 168 out of 200 [1]
group 2 also controls for the effect of being injected at all, since the mice received sterile salt solution [1]
group 3 shows that the weakened pathogen in the vaccine does not itself cause the disease [1]
⚠ If you missed marks here: Each control removes one alternative explanation, so say which one. “It is the control” on its own is worth nothing; the mark is for what would otherwise remain uncertain.
(b)[3]
Explain why the mice in group 1 were protected.
Model Answer — 6b
the vaccine contained weakened pathogens or their antigens, which stimulated the mice’s lymphocytes [1]
the lymphocytes produced antibodies with a shape complementary to the antigens, and also memory cells [1]
when the bacterium was met, antibodies were produced rapidly and in large quantity, so the bacteria were destroyed before they multiplied enough to cause illness [1]
⚠ If you missed marks here: Naming the cells is what earns the marks. “The vaccine made them immune” restates the result rather than explaining it, and cannot score any of the three.
(c)[2]
Group 4 was exposed again three months later without any further treatment, and 161 of them became ill. Explain this result.
Model Answer — 6c
group 4 had passive immunity — antibodies acquired from another individual — which protected them immediately but only while the antibodies lasted [1]
the antibodies were broken down and no memory cells were produced, because no antigen had entered to stimulate their lymphocytes, so there was nothing to replace them [1]
⚠ If you missed marks here: The comparison with group 1, which was still protected, is the point of the whole experiment. Passive immunity is the fast one and the short-lived one, and both properties come from the same fact.
(d)[4]
Evaluate this experiment as evidence that the vaccine would protect humans, and suggest two improvements to the investigation.
Model Answer — 6d
the difference is very large — 7 out of 200 against 168 out of 200 — and the groups were matched for age, sex, mass and conditions, so the vaccine is very likely to be the cause [1]
but mice are a different species and may respond differently from humans, and the dose that protects a mouse may not protect a person [1]
improvement: expose the mice to the pathogen again months later to test how long the protection lasts [1]
improvement: repeat with more groups at different doses, or repeat the whole experiment, to check the result is reproducible [1]
⚠ If you missed marks here: “Only 200 mice” is a weak criticism — 200 is a reasonable sample and the difference is enormous. The real limitations are the species and the fact that duration of protection was never tested for the vaccinated group.
Question 7 — Putting the Whole Topic Together
Total: 10 marks
A bacterial disease of a coastal town is transmitted when people eat shellfish taken from water into which untreated sewage flows. Some cases also occur among hospital staff after contact with patients’ blood. A vaccine exists but is expensive.
(a)[2]
Classify each of the two routes described as direct or indirect transmission, giving a reason for each.
Model Answer — 7a
shellfish — indirect, through contaminated food [1]
contact with patients’ blood — direct, through blood and other body fluids with nothing in between [1]
⚠ If you missed marks here: Apply one test to each route: was there anything in between? The shellfish is an intermediate; blood passing from one host into another is not.
(b)[3]
State three body defences that would act against this bacterium if a person ate contaminated shellfish or was splashed in the eye, and explain how each works.
Model Answer — 7b
stomach acid — kills most pathogens swallowed with food and drink [1]
skin — a continuous physical barrier that prevents entry while it is unbroken [1]
white blood cells — phagocytes engulf and digest any bacteria that get in; lymphocytes produce antibodies against them [1]
⚠ If you missed marks here: Only defences from the syllabus list of five will be credited, so tears are not an acceptable answer even though the question mentions the eye. Each mark also needs the mechanism, not only the name.
(c)[3]
The town council can afford only one measure. Suggest which measure would reduce cases most, and justify your choice using the information given.
Model Answer — 7c
sewage treatment, so that untreated sewage no longer flows into the water where the shellfish grow [1]
this closes the route responsible for most of the cases, since only some cases occur in hospital staff [1]
it protects the whole town at once, including anyone a vaccination programme would miss, and is likely to prevent other diseases carried in sewage as well [1]
⚠ If you missed marks here: The measure must be matched to the route carrying most cases, and the stem tells you which that is. Choosing the vaccine is defensible only if you argue it well, but it costs more and leaves the contaminated shellfish exactly as they were.
(d)[2]
Hospital staff who have recovered from this disease rarely catch it again, but they can still catch influenza. Explain why.
Model Answer — 7d
recovering produced memory cells against the antigens of this bacterium, so a second exposure produces antibodies rapidly and the person does not become ill [1]
the influenza virus has different antigens, so those antibodies and memory cells are not complementary to it and give no protection [1]
⚠ If you missed marks here: Both halves come from the same idea: specificity is about the shape of the antigen. Note that memory cells are not used up by acting, so “their immunity ran out” explains nothing here.
Self-Assessment
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