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IGCSE Biology Paper 4 (Theory / Extended)

Topic 10: Diseases and Immunity -- Challenge Exam 1
1 hour 15 minutes
80
7
75:00
0610

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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 — Pathogens, Transmission and the Body’s Own Barriers
Total: 12 marks
(a) [3]
Define a pathogen and a transmissible disease, and explain why rickets is not a transmissible disease.
Model Answer — 1a
a pathogen is a disease-causing organism [1]
a transmissible disease is one in which the pathogen can be passed from one host to another [1]
rickets is caused by a shortage of vitamin D, so there is no pathogen to pass on [1]
⚠ If you missed marks here: The word organism is doing the work in the first definition — a toxin and an antigen are both made by pathogens and neither is one. “Germ” appears on no mark scheme.
(b) [4]
For each of the following, state whether transmission is direct or indirect and give a reason: (i) a nurse pricked by a needle just used on an infected patient; (ii) a child who drinks water containing sewage; (iii) a person who breathes in droplets from a sneeze; (iv) a farmer bitten by an infected tick.
Model Answer — 1b
(i) direct — blood or other body fluid passes from one host to the other with nothing in between [1]
(ii) indirect — through contaminated water or food [1]
(iii) indirect — through the air [1]
(iv) indirect — an animal acts as a vector [1]
⚠ If you missed marks here: The sneeze is the one that goes wrong. Closeness does not make transmission direct; the syllabus lists the air as an indirect route. The only question that decides it is whether anything came in between.
(c) [3]
State three body defences named in the syllabus and describe how each one works.
Model Answer — 1c
any three from: skin — a continuous physical barrier while unbroken [1]
hairs in the nose — trap dust and larger particles in the air; mucus — traps pathogens which are then swept away [1]
stomach acid — kills most pathogens taken in with food and drink; white blood cells — phagocytes engulf and digest pathogens, lymphocytes produce antibodies [1]
⚠ If you missed marks here: The syllabus says body defences are limited to five, so tears, sweat, earwax and coughing all score zero however true they are. Each mark also needs the mechanism, not just the name.
(d) [2]
Explain why a patient with a large burn is at much greater risk of infection.
Model Answer — 1d
the skin is a physical barrier that only works while it is continuous or unbroken [1]
a burn removes that barrier over a large area, so pathogens can enter the body and reach the blood directly [1]
⚠ If you missed marks here: This is a barrier question, not an immunity question. The patient’s lymphocytes and phagocytes are unchanged — what has changed is that a doorway has been opened, which is also why clotting is described as preventing the entry of pathogens.
Question 2 — Antigens, Antibodies and the Two White Blood Cells
Total: 12 marks
(a) [3]
Explain what is meant by an antigen and by an antibody. Your answer must make clear where each is found and what produces it.
Model Answer — 2a
an antigen is a molecule on the surface of a pathogen [1]
an antibody is a protein [1]
produced by a lymphocyte in the body [1]
⚠ If you missed marks here: This part is written so that swapping the two words loses all three marks at once, which is exactly why examiners write it this way. Before you use either word, ask where is it and who made it.
(b) [3]
Explain what is meant by saying that antibodies are specific, and use this to explain why a person immune to measles can still catch cholera.
Model Answer — 2b
each pathogen has its own antigens, which have specific shapes [1]
an antibody has a shape complementary to one antigen and fits no other [1]
the cholera bacterium has different antigens, so the measles antibodies cannot bind to them and give no protection [1]
⚠ If you missed marks here: Cambridge marks the word complementary and refuses “the same shape”. Note also that this is a statement about shape, never about how strong or healthy someone’s immune system is.
(c) [4]
Name the two types of white blood cell involved in defence, describe what each does, and explain how they can work together against the same pathogen.
Model Answer — 2c
phagocyte: engulfs and digests pathogens by phagocytosis [1]
lymphocyte: produces antibodies [1]
antibodies bind to the antigens on the pathogen [1]
this may mark the pathogen for destruction by phagocytes, which then engulf it — so the lymphocyte labels and the phagocyte removes [1]
⚠ If you missed marks here: “White blood cells eat the antigens” contains two errors: eat is not a marking term, and what is engulfed is the whole pathogen. The cooperation mark is the one most often missed, and it is stated explicitly in the syllabus.
(d) [2]
State the two things that may happen to a pathogen once an antibody has bound to its antigens.
Model Answer — 2d
direct destruction of the pathogen [1]
marking of the pathogen for destruction by phagocytes [1]
⚠ If you missed marks here: Two outcomes, two marks, and candidates who write only “the antibody kills it” can score at most one. The second outcome is the sentence that ties this sub-topic back to the white blood cells of 10.2.
Question 3 — Vaccination, and What It Does to a Population
Total: 12 marks
(a) [4]
Outline the process of vaccination, from the injection to long-term immunity.
Model Answer — 3a
weakened pathogens or their antigens are put into the body [1]
the antigens stimulate an immune response by lymphocytes [1]
the lymphocytes produce antibodies [1]
memory cells are produced, which give long-term immunity [1]
⚠ If you missed marks here: The order is antigen, lymphocyte, antibody, memory cell, and the memory cell step is the one candidates leave off when they are running short of time. Any answer beginning “antibodies are injected” has already lost the first mark and usually the rest.
(b) [3]
Explain why a vaccine produces immunity without producing the disease, and why this makes vaccination preferable to gaining immunity by catching the disease.
Model Answer — 3b
the pathogen in the vaccine is weakened, so it cannot cause the disease [1]
it still carries the same antigens, which are what stimulate the lymphocytes [1]
so the same antibodies and the same memory cells are produced without the person becoming ill [1]
⚠ If you missed marks here: The mark is for the mechanism, not for the opinion that vaccination is safer. The key idea is that weakening removes the ability to cause disease while leaving the antigens intact — if it destroyed the antigens the vaccine would do nothing at all.
(c) [3]
Explain how vaccinating a large proportion of a population protects a newborn baby who is too young to be vaccinated.
Model Answer — 3c
vaccinated people do not develop the disease, so the pathogen does not multiply in them [1]
so they cannot transmit the pathogen to anyone else [1]
with a large enough proportion vaccinated the pathogen cannot spread through the population, so it is very unlikely to reach the baby [1]
⚠ If you missed marks here: This is a transmission answer, not an immunity answer. Any version in which antibodies or immunity pass between people is describing something that does not happen — antibodies move between individuals only across the placenta and in breast milk.
(d) [2]
In a village where the well is contaminated with sewage, a health worker argues that treating the water would reduce cholera more than vaccinating the villagers. Suggest two reasons why she may be right.
Model Answer — 3d
cholera is transmitted in contaminated water, so treating the water closes the transmission route for everyone at once, including anyone the vaccination programme misses [1]
villagers drinking from the well are exposed repeatedly and in large doses, and clean water also prevents other water-borne diseases [1]
⚠ If you missed marks here: The route named in the stem is the key to every applied question in this topic. Vaccination raises the defences of individuals; a clean water supply removes the pathogen before it ever reaches anyone.
Question 4 — Reading an Antibody Graph
Total: 12 marks
A volunteer was injected with antigen P on day 0 and again on day 30. The antibody concentration in the blood was measured, in arbitrary units.
Day06122030344070
Antibody concentration / arbitrary units021062479072
(a) [2]
Calculate how many times greater the highest antibody concentration after the second injection is than the highest concentration after the first injection. Show your working.
Model Answer — 4a
highest after the first injection = 10; highest after the second = 90 [1]
90 ÷ 10 = 9 times greater [1]
⚠ If you missed marks here: Take the peak of each response, not the first or last reading. Show the working: the method mark is awarded separately from the final value, so a bare wrong number scores nothing at all.
(b) [3]
Describe three ways in which the response to the second injection differs from the response to the first. Use figures from the table.
Model Answer — 4b
faster — 47 units only 4 days after the second injection, whereas only 10 units had been reached 12 days after the first [1]
larger — peak 90 units against 10 units [1]
longer lasting — still 72 units at day 70, whereas the first response had fallen to 2 units by day 30 [1]
⚠ If you missed marks here: Three separate differences means three separate marks, so merging them into “the second response was much better” costs two. A describe question wants the shape and the numbers, not the cells.
(c) [3]
Explain the difference between the two responses.
Model Answer — 4c
memory cells were produced during the first response [1]
they recognise the same antigen immediately, so there is little or no lag [1]
antibodies are therefore produced faster and in far greater quantity [1]
⚠ If you missed marks here: One cause explains all three differences, so this is not a request for three different reasons. If your explain answer contains only numbers, you have answered the describe question twice.
(d) [4]
A second volunteer was instead given an injection of antibodies against antigen P taken from a person who had recovered. Name the type of immunity this produces, describe how the graph for this volunteer would differ, and explain why.
Model Answer — 4d
passive immunity — antibodies acquired from another individual [1]
the concentration would be at its highest immediately, with no lag and no rise [1]
it would then fall steadily to zero over weeks or months [1]
because the antibodies are broken down and no memory cells are produced — no antigen entered, so the lymphocytes were never stimulated [1]
⚠ If you missed marks here: Read the value at time zero first: a curve that starts at its maximum can only be antibodies made by somebody else. The absence of memory cells is the reason the curve never recovers.
Question 5 — A Baby’s First Eighteen Months
Total: 10 marks
The concentration of measles antibodies in a baby’s blood was measured. The baby was breastfed until 7 months and vaccinated against measles at 12 months.
Age / months0369121318
Antibody concentration / arbitrary units865827405244
(a) [2]
Name the type of immunity responsible for the antibodies present at birth, and state two ways in which those antibodies could have reached the baby.
Model Answer — 5a
passive immunity [1]
across the placenta before birth, and in breast milk after birth [1]
⚠ If you missed marks here: A concentration that is highest at the very start, with no rise before it, can only be antibodies made by someone else. Both named routes involve the mother, and only one of them is available before birth.
(b) [3]
Explain why the antibody concentration had fallen to zero by 12 months.
Model Answer — 5b
antibodies are proteins and are gradually broken down [1]
no antigen entered the baby, so the baby’s lymphocytes were never stimulated [1]
so no memory cells were produced and the baby cannot make replacements [1]
⚠ If you missed marks here: Saying only that the antibodies were broken down is one mark of three. The examiner wants to know why nothing replaced them, and that answer follows straight from the definition of passive immunity.
(c) [3]
Explain why the protection produced by the vaccination at 12 months will last far longer than the protection the baby had at birth.
Model Answer — 5c
the vaccine contains antigens, which stimulate the baby’s own lymphocytes to produce antibodies — this is active immunity [1]
memory cells are also produced and remain in the body [1]
so if the measles virus is met later, antibodies are produced faster and in greater quantity and the child does not become ill [1]
⚠ If you missed marks here: Antibodies themselves are broken down within weeks, so they cannot be what makes protection last for years. Only the memory cells can, and an answer that never mentions them cannot score this part.
(d) [2]
Suggest why health services regard the period between 9 and 12 months as a time of particular risk for this baby.
Model Answer — 5d
the passive protection from the mother has almost gone — only 4 units at 9 months and 0 at 12 [1]
but the baby has not yet been vaccinated, so it has no antibodies and no memory cells of its own against measles [1]
⚠ If you missed marks here: Use the figures in the table rather than describing the gap in general terms. This gap is the whole reason vaccination schedules are timed the way they are, and it is a favourite suggest question.
Question 6 — Cholera, From the Well to the Blood
Total: 12 marks
(a) [2]
State the type of organism that causes cholera and how it is transmitted.
Model Answer — 6a
a bacterium [1]
transmitted in contaminated water [1]
⚠ If you missed marks here: Both facts are named in the syllabus and both are one-word marks. Calling it a virus also makes the rest of the mechanism impossible, since the answer depends on a bacterium multiplying in the gut and releasing a toxin.
(b) [4]
Explain how the cholera bacterium causes diarrhoea. Use the term water potential in your answer.
Model Answer — 6b
the bacterium in the small intestine produces a toxin [1]
the toxin causes the cells lining the small intestine to secrete chloride ions into the small intestine [1]
the gut contents therefore have a lower water potential than the cells and blood [1]
so water moves into the gut by osmosis, through partially permeable membranes, producing watery faeces [1]
⚠ If you missed marks here: Four marks means four links and none may be skipped. Writing that the toxin “draws water out of the blood” deletes the chloride ion mark and the osmosis mark in a single phrase — nothing moves the water, it moves down a gradient the ions created.
(c) [3]
A patient with cholera is treated with a drink containing water, glucose and ions rather than pure water. Explain why.
Model Answer — 6c
the patient has lost large volumes of water, causing dehydration [1]
ions have been lost from the blood as well as water [1]
replacing water alone would leave the blood plasma too dilute, so the ions must be replaced too [1]
⚠ If you missed marks here: The treatment mirrors the two named consequences exactly, so knowing both of them pays off twice. An answer that mentions only dehydration cannot explain why the drink is not simply water.
(d) [3]
State three measures that would reduce the spread of cholera in a village, and for each explain how it works.
Model Answer — 6d
a clean water supply — treated water contains no pathogens, so the bacterium is not swallowed [1]
sewage treatment — stops bacteria from the faeces of infected people reaching wells, rivers and crops [1]
hygienic food preparation or good personal hygiene — hands and food are not contaminated, so the bacterium is not carried to the mouth [1]
⚠ If you missed marks here: Each mark needs the measure and the reason. Note also that a clean water supply and sewage treatment are two different answers — one deals with what reaches people and the other with what leaves them.
Question 7 — An Unfamiliar Disease
Total: 10 marks
A viral disease of farm workers spreads in three ways: workers are bitten by infected fruit bats; workers drink raw palm sap that bats have fed on overnight; and family members are occasionally infected through contact with a patient’s saliva. No vaccine exists.
(a) [3]
Classify each of the three routes as direct or indirect transmission, giving a reason for each.
Model Answer — 7a
bat bite — indirect, an animal acts as a vector [1]
raw sap — indirect, through contaminated food or drink [1]
saliva from a patient — direct, a body fluid passing with nothing in between [1]
⚠ If you missed marks here: Do not be led by which route sounds most dangerous. Apply one test three times: was there anything in between? An animal, food and water are all intermediates; a body fluid passing straight from one host to another is not.
(b) [3]
A worker who recovers from this disease does not catch it again. Explain why, naming the cells and molecules involved.
Model Answer — 7b
during the infection, lymphocytes were stimulated by the antigens on the virus and produced antibodies with a complementary shape [1]
memory cells were also produced and remain in the body [1]
on a second exposure the memory cells produce antibodies faster and in greater quantity, so the virus is destroyed before symptoms develop [1]
⚠ If you missed marks here: Naming the cells is what turns this from a two-mark answer into a three-mark one. “Their immune system remembers it” is a description of the effect with none of the mechanism the mark scheme is paying for.
(c) [2]
Suggest, with reasons, the two most effective control measures for this disease.
Model Answer — 7c
stop people drinking raw sap, or cover the collecting pots so bats cannot reach them — this closes the contaminated food and drink route [1]
use gloves and wash hands when caring for a patient — this closes the direct body-fluid route [1]
⚠ If you missed marks here: Each measure must be paired with the route it closes, and vaccination is excluded by the stem. A list of general hygiene advice with no routes attached scores badly however sensible it sounds.
(d) [2]
A researcher proposes injecting exposed family members with antibodies taken from recovered patients. State the type of immunity this would give and one advantage and one disadvantage compared with a vaccine.
Model Answer — 7d
passive immunity; the advantage is that protection is immediate, which matters for someone already exposed [1]
the disadvantage is that it is short-term, because the antibodies are broken down and no memory cells are produced [1]
⚠ If you missed marks here: The two properties of passive immunity are the opposite way round from the way the word sounds: it is the fast one and the short-lived one. Both halves of the comparison come from the same fact, the absence of memory cells.

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IGCSE Biology 0610 — Paper 4 Theory (Extended) — Topic 10: Diseases and Immunity — Challenge Exam 1
For examination preparation only. Not an official Cambridge Assessment paper.