IGCSE Biology Paper 4 (Theory / Extended)

Topic 10: Diseases and Immunity -- Challenge Exam 3

Duration1 hour 15 minutes

Total Marks80

Questions7

Time Remaining75:00

Syllabus0610

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 — Definitions, Routes and the Limits of a List

Total: 12 marks

(a) [2]

Define a transmissible disease and give one example of a disease that is not transmissible, with a reason.

Model Answer — 1a

a disease in which the pathogen can be passed from one host to another [1]

e.g. scurvy or rickets, because it is caused by a shortage of a nutrient and there is no pathogen to pass on [1]

⚠ If you missed marks here: The example must come with the reason. Note also that a disease can be severe, common and preventable and still not be transmissible — the test is only whether an organism travels between hosts.

1 mark — pathogen can be passed from one host to another 1 mark — named non-transmissible disease with the reason that there is no pathogen

(b)(i) [2]

State what is meant by direct transmission and give one example.

Model Answer — 1b

the pathogen passes from one host to another with nothing in between, including through blood and other body fluids [1]

example: a shared needle, a blood transfusion, sexual contact, or a pathogen crossing the placenta [1]

⚠ If you missed marks here: The phrase the syllabus prints is blood and other body fluids, so it is worth using. An answer offering a sneeze here has classified the commonest airborne route the wrong way round.

1 mark — passes from host to host with nothing in between, or via body fluids 1 mark — a correct example of direct transmission

(b)(ii) [4]

State the four kinds of indirect transmission named in the syllabus, and give an example of each.

Model Answer — 1c

from contaminated surfaces — e.g. a door handle or a shared cup [1]

from contaminated food or water — e.g. food handled with unwashed hands, or water containing sewage [1]

from animals — e.g. a mosquito or a housefly acting as a vector [1]

from the air — e.g. droplets breathed out in a cough or a sneeze [1]

⚠ If you missed marks here: Four routes, four marks. Candidates who have learned only “touching things and eating bad food” lose half of this part, and the air is the one most often forgotten because it feels like direct transmission.

1 mark — contaminated surfaces, with an example 1 mark — contaminated food or water, with an example 1 mark — animals or vectors, with an example 1 mark — the air, with an example

A student writes that the body’s defences include tears, sweat and coughing. Explain why this answer would score no marks in an examination, and give two defences that would.

Model Answer — 1d

the syllabus states that body defences are limited to skin, hairs in the nose, mucus, stomach acid and white blood cells, so anything outside that list is not credited [1]

any two from that list, e.g. mucus traps pathogens which are swept away, and stomach acid kills most pathogens swallowed with food [1]

⚠ If you missed marks here: “Limited to” appears several times in the 0610 syllabus and always means a closed list. Knowing extra biology is good; spending exam time on it is not.

1 mark — the syllabus limits body defences to a list of five 1 mark — two defences from the list, with how they work

(d) [2]

Explain why identifying the transmission route of a new disease is the most useful first step for a public health team.

Model Answer — 1e

every control measure works by closing a transmission route, so the route determines which measure will work [1]

e.g. a water-borne disease needs a clean water supply and sewage treatment, whereas an airborne disease would not be affected by either [1]

⚠ If you missed marks here: A worked example is what turns a general statement into the second mark. Severity and death rates tell you how urgent a problem is but nothing at all about what to do first.

1 mark — control measures work by closing a route, so the route decides the measure 1 mark — a supporting example contrasting two routes

Question 2 — The First Time a Pathogen Gets In

Total: 12 marks

(a) [3]

A bacterium enters the blood through a cut. Describe how phagocytes act against it, and explain why they can do so even though the body has never met this bacterium before.

Model Answer — 2a

the phagocyte engulfs the bacterium, taking it inside the cell [1]

and digests it — the process is called phagocytosis [1]

phagocytes act against pathogens generally and do not need a complementary shape or any previous exposure [1]

⚠ If you missed marks here: “Eats” is not a marking term and neither is “kills”. The third mark is the one that explains why phagocytes are grouped with the barriers rather than with immunity.

1 mark — engulfs the pathogen 1 mark — digests it; the process is phagocytosis 1 mark — acts generally, needing no previous exposure

(b) [4]

Describe how lymphocytes act against the same bacterium, naming the molecules involved and explaining what happens to the bacterium as a result.

Model Answer — 2b

the antigens on the surface of the bacterium stimulate lymphocytes [1]

the lymphocytes produce antibodies, which are proteins with a shape complementary to those antigens [1]

the antibodies bind to the antigens [1]

the bacterium is then destroyed directly, or marked for destruction by phagocytes [1]

⚠ If you missed marks here: The lymphocyte makes a molecule; it does not engulf anything. The final mark links the two white blood cells together, and it is the one most often left off — the antibody labels, the phagocyte removes.

1 mark — antigens on the pathogen stimulate lymphocytes 1 mark — lymphocytes produce antibodies with a complementary shape 1 mark — antibodies bind to the antigens 1 mark — pathogen destroyed directly or marked for phagocytes

The antibody concentration in this person’s blood is zero for the first five days after infection, rises to a peak on day 14 and has almost disappeared by day 40. Explain the shape of this graph.

Model Answer — 2c

at first the right lymphocytes have not yet been stimulated by the antigen and have not begun producing antibodies, so there is a lag [1]

once stimulated, the lymphocytes produce antibodies, so the concentration rises to a peak [1]

antibodies are proteins and are broken down; once the pathogen has been destroyed, fewer are produced, so the concentration falls [1]

⚠ If you missed marks here: This is a primary response, and all three features — the lag, the low peak and the fall — are normal and expected. The fall does not mean the person has lost immunity, because the memory cells remain behind.

1 mark — lag while lymphocytes are stimulated and begin producing antibodies 1 mark — rise as lymphocytes produce antibodies 1 mark — fall as antibodies are broken down and fewer are produced

(d) [2]

Explain why this person is protected against the same bacterium years later even though almost no antibodies remain in the blood.

Model Answer — 2d

memory cells produced during the first response remain in the body [1]

on a second exposure they recognise the same antigen at once and antibodies are produced faster and in far greater quantity, so the bacterium is destroyed before symptoms appear [1]

⚠ If you missed marks here: Antibodies do not last for years, which is exactly why this question is asked. Only memory cells can explain long-term immunity, and an answer that says the antibodies “stay in the blood” contradicts the data in part (c).

1 mark — memory cells remain in the body 1 mark — a faster, larger response destroys the pathogen before symptoms

Question 3 — Two Kinds of Injection

Total: 12 marks

Two groups of adults were treated against the same disease. Group A received a vaccine on day 0. Group B received an injection of antibodies taken from recovered patients on day 0. Antibody concentration in the blood was measured, in arbitrary units.

Day	0	3	10	21	60	120
Group A	0	0	18	41	36	30
Group B	72	63	44	21	2	0

(a) [2]

State which group gained active immunity and which gained passive immunity, giving a reason based on the data.

Model Answer — 3a

group A gained active immunity — the concentration starts at zero and rises, so the antibodies were produced by the person’s own lymphocytes [1]

group B gained passive immunity — the concentration is highest at day 0 with no rise at all, so the antibodies came from another individual [1]

⚠ If you missed marks here: Read the value at time zero before anything else. A curve that begins at its maximum can only be antibodies made by someone else, and that single observation answers this part on its own.

1 mark — group A active, because the concentration starts at zero and rises 1 mark — group B passive, because it is highest at day 0 with no rise

(b) [3]

Calculate the percentage decrease in group B between day 0 and day 21, and use the data to state one advantage and one disadvantage of the treatment given to group B.

Model Answer — 3b

72 − 21 = 51; 51 ÷ 72 × 100 = 70.8 % [1]

advantage: protection is immediate — 72 units on day 0, when group A still had none [1]

disadvantage: protection is short-term — it reaches zero by day 120, whereas group A still had 30 units [1]

⚠ If you missed marks here: Percentage change is change divided by the original. For the advantage and disadvantage, quote the figures: an answer that describes passive immunity in general without touching the table has ignored the command.

1 mark — 70.8 % decrease, with working 1 mark — advantage: immediate protection, using figures 1 mark — disadvantage: short-term, using figures

Explain the shape of each curve in terms of the cells and molecules involved.

Model Answer — 3c

group A: the vaccine contains antigens, which must first stimulate lymphocytes, so there is a lag before antibodies appear [1]

group A: memory cells are produced as well as antibodies, so a raised concentration is maintained and protection is long-term [1]

group B: the antibodies were already made, so the concentration is at its highest immediately [1]

group B: the antibodies are gradually broken down and no memory cells were produced, because no antigen entered to stimulate the lymphocytes, so nothing replaces them [1]

⚠ If you missed marks here: Each curve needs two ideas, one for its start and one for what happens afterwards. An answer that explains only why passive immunity is fast has answered half the question.

1 mark — group A lag: antigens must stimulate lymphocytes first 1 mark — group A maintained: memory cells produced, long-term 1 mark — group B immediate: antibodies already made 1 mark — group B falls to zero: broken down, no memory cells to replace them

(d) [3]

A doctor must treat a patient who was exposed to the disease yesterday and a healthy traveller leaving in three months. Suggest which treatment each should receive and justify both choices.

Model Answer — 3d

the exposed patient should receive the antibody injection (passive) [1]

because protection is immediate and there is no time to wait for the patient’s own lymphocytes to respond [1]

the traveller should receive the vaccine (active), because there is time for antibodies and memory cells to be produced, giving protection that lasts throughout the trip and beyond [1]

⚠ If you missed marks here: This question rewards understanding rather than recall, and it is the one place where the “inferior” kind of immunity is the right answer. Match the property — speed or duration — to what the patient actually needs.

1 mark — exposed patient: antibody injection or passive immunity 1 mark — justification: immediate protection, no time to wait 1 mark — traveller: vaccine, because memory cells give lasting protection

Question 4 — Cholera in Detail

Total: 12 marks

A patient admitted with cholera lost 12.5 dm³ of watery faeces in 24 hours. Her body mass fell from 58.0 kg to 51.2 kg. The fluid lost contained chloride ions at 106 mmol per dm³; normal blood plasma contains about 100 mmol per dm³.

(a) [2]

Calculate the percentage loss in body mass. Show your working.

Model Answer — 4a

58.0 − 51.2 = 6.8 kg [1]

6.8 ÷ 58.0 × 100 = 11.7 % [1]

⚠ If you missed marks here: Divide by the mass before the illness, not after. Dividing by 51.2 gives 13.3 %, which is wrong, and without working there is no method mark to fall back on.

1 mark — loss of 6.8 kg calculated 1 mark — 11.7 %, with working shown

(b) [4]

Explain, in terms of water potential, why the patient lost such a large volume of watery fluid.

Model Answer — 4b

the cholera 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 small intestine [1]

this gives the gut contents a lower water potential than the cells and the blood [1]

water therefore moves from a higher to a lower water potential into the gut, by osmosis, through partially permeable membranes [1]

⚠ If you missed marks here: Four links and none may be skipped. Writing that the toxin “pulls water out of the blood” loses the ion mark and the osmosis mark together — nothing pulls the water, it moves down a gradient the ions created.

1 mark — the bacterium produces a toxin 1 mark — chloride ions secreted into the small intestine 1 mark — gut contents have a lower water potential than the blood 1 mark — water moves in by osmosis through partially permeable membranes

Using the data given, explain why the patient is treated with a drink containing water and ions rather than with pure water.

Model Answer — 4c

the fluid lost contains chloride at 106 mmol per dm³, slightly more concentrated than plasma at 100, so ions have been lost as well as water [1]

over 12.5 dm³ that is a very large loss of ions from the blood [1]

replacing the water alone would leave the blood plasma too dilute, so the ions must be replaced as well [1]

⚠ If you missed marks here: The two figures are in the stem for a reason — quoting them turns a general answer into a data answer. Note that loss of ions from the blood is one of the two consequences the syllabus names, and it carries its own mark.

1 mark — the fluid contains ions at a concentration similar to plasma, quoting figures 1 mark — a very large quantity of ions lost over 12.5 dm³ 1 mark — water alone would leave the plasma too dilute

(d) [3]

The patient’s village takes drinking water from a river and has no latrines. Explain how two named measures would reduce future cases, and state which you would introduce first, with a reason.

Model Answer — 4d

a clean water supply — treated water contains no pathogens, so the bacterium is not swallowed [1]

sewage treatment or latrines — the bacterium leaves infected people in the faeces, and treating the waste stops it reaching the river [1]

a reasoned choice, e.g. treat the drinking water first because it protects everyone immediately, or build latrines first because it stops the river being contaminated in the first place [1]

⚠ If you missed marks here: Cholera transmission is a loop from faeces to water to mouth, and these two measures close its two halves. The third mark is for a justified choice; either answer is acceptable if the reasoning is sound.

1 mark — clean water supply, with the reason 1 mark — sewage treatment or latrines, with the reason 1 mark — a reasoned choice of which to introduce first

Question 5 — Why Immunity Does Not Transfer

Total: 10 marks

(a) [2]

State what is meant by active immunity and give the two ways in which it may be gained.

Model Answer — 5a

defence against a pathogen by antibody production in the body [1]

gained after an infection by a pathogen, or by vaccination [1]

⚠ If you missed marks here: The word active describes whose lymphocytes did the work, not how quickly protection appears. Both ways of gaining it produce the same antibodies and the same memory cells.

1 mark — defence by antibody production in the body 1 mark — after infection by a pathogen, or by vaccination

(b) [3]

A patient is immune to measles. Explain why this gives no protection at all against the cholera bacterium, even though antibodies from the measles infection are still present in the blood.

Model Answer — 5b

each pathogen has its own antigens, which have specific shapes [1]

an antibody has a shape complementary to one antigen only [1]

the measles antibodies are not complementary to the antigens on the cholera bacterium, so they cannot bind to them and the bacterium is unaffected [1]

⚠ If you missed marks here: This is a statement about shape, never about strength. If your answer would still make sense with the words “tired” or “run down” in it, it is not answering the question that was asked.

1 mark — each pathogen has its own antigens with specific shapes 1 mark — an antibody is complementary to one antigen only 1 mark — measles antibodies cannot bind to cholera antigens

Explain why vaccination is preferable to gaining immunity by catching a disease, and why a vaccine does not make the person ill.

Model Answer — 5c

the pathogen in the vaccine is weakened, so it cannot cause the disease [1]

it still carries the same antigens, so it stimulates the same immune response [1]

the person therefore produces the same antibodies and the same memory cells, gaining long-term immunity without the risks of the illness [1]

⚠ If you missed marks here: The mark is for the mechanism, not the opinion. Weakening removes the ability to cause disease while leaving the antigens intact, and it is the antigens the lymphocytes respond to.

1 mark — the pathogen is weakened and cannot cause the disease 1 mark — it still carries the same antigens 1 mark — the same antibodies and memory cells result, without the illness

(d) [2]

A student says that a person who has had many infections has a stronger immune system and is less likely to catch anything new. Comment on this statement.

Model Answer — 5d

they will have memory cells against the pathogens they have met, so they are protected against those diseases [1]

but immunity is specific, so having met many antigens gives no protection against a pathogen with different antigens — a new disease produces a slow primary response however experienced the person is [1]

⚠ If you missed marks here: Be fair to the statement before correcting it: the first half of it is true. The error is treating immunity as a general level of fitness rather than as a collection of specific shapes.

1 mark — protected against the pathogens already met, because of memory cells 1 mark — but immunity is specific, so a new antigen gives a primary response

Question 6 — Judging a Public Health Claim

Total: 12 marks

Two neighbouring districts introduced different measures against a bacterial gut disease. District P built latrines and a sewage works. District Q installed treated piped drinking water. Cases per 10 000 people are shown before and after.

District	Cases before	Cases after
P (sewage treated)	186	84
Q (piped treated water)	174	71
P and Q, both measures in place two years later	—	6

(a) [2]

Calculate the percentage decrease in cases in district Q after the piped water was installed. Show your working.

Model Answer — 6a

174 − 71 = 103 [1]

103 ÷ 174 × 100 = 59.2 % [1]

⚠ If you missed marks here: Change divided by the original, and the original is the “before” figure. Showing the working protects the method mark if the arithmetic slips.

1 mark — change of 103 calculated 1 mark — 59.2 % decrease, working shown

(b) [4]

Explain why each measure on its own reduced cases by only about half, but the two together reduced them by more than 95 per cent.

Model Answer — 6b

sewage treatment stops the bacterium leaving infected people and reaching the water, but district P was still drinking untreated water contaminated from elsewhere [1]

treated drinking water stops the bacterium reaching people who drink it, but district Q was still releasing untreated sewage that contaminated rivers, crops and hands [1]

transmission is a loop: from the faeces of an infected person, into the water or food, into the mouth of the next person [1]

each measure alone leaves one half of the loop open so transmission continues; closing both halves breaks the cycle rather than slowing it [1]

⚠ If you missed marks here: This is why the syllabus lists a clean water supply and sewage treatment as two different measures. An answer that treats them as the same idea cannot explain why the combined effect is so much greater than the sum of the two.

1 mark — sewage treatment deals with what leaves people, but drinking water was still untreated 1 mark — treated water deals with what reaches people, but sewage was still released 1 mark — transmission is a loop from faeces to water or food to mouth 1 mark — each alone leaves half the loop open; both together break the cycle

Explain how three further measures could reduce the remaining cases, naming each measure and the route it closes.

Model Answer — 6c

hygienic food preparation — washing food and surfaces and cooking thoroughly closes the contaminated food route [1]

good personal hygiene — hand washing, especially after using the toilet and before handling food, closes the hand and surface route [1]

waste disposal — removing rubbish removes the breeding sites of flies and rats, which act as vectors [1]

⚠ If you missed marks here: Each measure must be paired with the route it closes. A list of hygiene advice with no routes attached reads well and scores badly, because the command word is explain.

1 mark — hygienic food preparation, with the route it closes 1 mark — good personal hygiene, with the route it closes 1 mark — waste disposal, with the route it closes

(d) [3]

A newspaper claims the fall was caused by people becoming naturally immune to the bacterium over the two years. Evaluate this claim using the data.

Model Answer — 6d

it is true that people who recover produce antibodies and memory cells, so some individuals would become immune [1]

but the timing does not support the claim — the falls happened immediately after each measure was introduced, and the two districts fell at different times matching their own measures [1]

immunity is also specific to the person, and would not explain a fall of over 95 % across two whole districts, so the control measures are much the better explanation [1]

⚠ If you missed marks here: Evaluate means giving the claim its due and then testing it against the evidence. The strongest single point here is timing: the falls line up with the engineering, not with the passage of time.

1 mark — acknowledges that recovery does produce memory cells and immunity 1 mark — timing of the falls matches the measures introduced 1 mark — a reasoned conclusion that the measures are the better explanation

Question 7 — One Last Unfamiliar Disease

Total: 10 marks

A disease of villagers in a forest region is caused by a protoctist. It is spread by sandflies that bite infected dogs and then bite people, and occasionally by transfusion of infected blood. There is no vaccine.

(a) [3]

Name the type of organism causing this disease, and classify each of the two routes as direct or indirect, giving a reason for each.

Model Answer — 7a

a protoctist [1]

sandfly — indirect, because an animal acts as a vector carrying the pathogen between hosts [1]

blood transfusion — direct, because blood passes from one host to another with nothing in between [1]

⚠ If you missed marks here: “Germ” is not an acceptable answer for the organism. For the routes, apply the same one-question test each time: was there anything in between?

1 mark — protoctist 1 mark — sandfly: indirect, animal vector 1 mark — transfusion: direct, blood with nothing in between

(b) [3]

Suggest three control measures for this disease and, for each, explain how it would work. You may not suggest vaccination.

Model Answer — 7b

control the sandflies, e.g. insecticide or nets — removes the vector, closing the indirect route [1]

treat or control infected dogs — removes the reservoir of pathogen the sandflies feed on [1]

screen and test donated blood — closes the direct blood-to-blood route [1]

⚠ If you missed marks here: Each measure must be matched to a route in the stem, and general advice such as “wash your hands” closes no route this pathogen uses. The stem rules out the vaccine, so an answer built on it scores nothing.

1 mark — a measure targeting the sandfly vector, with reason 1 mark — a measure targeting infected dogs, with reason 1 mark — screening donated blood, with reason

Explain what would happen inside a villager’s body when this protoctist entered for the first time, naming two cells and two molecules involved.

Model Answer — 7c

phagocytes engulf and digest pathogens by phagocytosis; the antigens on the protoctist stimulate lymphocytes [1]

the lymphocytes produce antibodies with a complementary shape, which bind to the antigens so the pathogen is destroyed or marked for phagocytes; memory cells are also produced [1]

⚠ If you missed marks here: The question tells you how many cells and molecules it wants, which is a strong hint about the mark scheme. Phagocyte, lymphocyte, antigen and antibody is the safest set to name.

1 mark — phagocytes engulf; antigens stimulate lymphocytes 1 mark — lymphocytes produce antibodies that bind; memory cells produced

(d) [2]

A doctor suggests giving antibodies from recovered villagers to people who have just been bitten. State the type of immunity this gives and one reason why it would not solve the problem for the village.

Model Answer — 7d

passive immunity — antibodies acquired from another individual, giving immediate protection [1]

but it is short-term: no memory cells are produced, so each person would need a fresh injection after every bite, and it does not close the transmission route for the village [1]

⚠ If you missed marks here: Two ideas in the second mark, and either will do: the absence of memory cells, or the fact that treating individuals never closes a route. Both are the arguments that run through the whole topic.

1 mark — passive immunity, immediate protection 1 mark — short-term with no memory cells, or does not close the transmission route

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