C3.2 Defence against disease
Practice exam-style IB Biology questions for Defence against disease, aligned with the syllabus and grouped by topic.
Question 1
What is a pathogen?
A.
A disease-causing organism or infectious agent
B.
A host cell infected by a virus
C.
An antibody secreted by a lymphocyte
D.
A symptom produced by tissue damage
Question 2
What is the role of mucus in defence against disease?
A.
It converts fibrinogen into fibrin at damaged blood vessels.
B.
It digests pathogens after endocytosis by phagocytes.
C.
It traps pathogens and particles on exposed epithelial surfaces.
D.
It produces antibodies specific to pathogen antigens.
Question 3
Which reaction is catalysed by thrombin during blood clotting?
A.
Fibrinogen is converted into fibrin.
B.
Antigens are converted into antibodies.
C.
Erythrocytes are converted into platelets.
D.
Fibrin is converted into fibrinogen.
Question 4
What happens to a pathogen after it is engulfed by a phagocyte?
A.
It releases clotting factors from platelets.
B.
It is converted into a memory cell.
C.
It secretes antibodies into the blood.
D.
It is digested by enzymes from lysosomes.
Question 5
State one type of pathogen that can infect humans.
[1]
State why archaea are not usually included as human pathogens.
[1]
Question 6
Which feature distinguishes adaptive immunity from innate immunity?
A.
Adaptive immunity is limited to phagocytes engulfing pathogens.
B.
Adaptive immunity responds only to broad categories of pathogens.
C.
Adaptive immunity does not change during an organism’s life.
D.
Adaptive immunity builds memory after exposure to specific antigens.
Question 7
What is an antigen in an adaptive immune response?
A.
A hydrolytic enzyme secreted by lysosomes in phagocytes
B.
A soluble clotting protein converted into fibrin
C.
A chemical that inhibits bacterial ribosome function
D.
A molecule recognized as foreign that can trigger antibody production
Question 8
Which mechanism can transmit HIV from one person to another?
A.
Sharing a classroom with an infected person
B.
Drinking water from a contaminated public pump
C.
Being bitten by a mosquito after it feeds on an infected person
D.
Sharing hypodermic needles contaminated with blood
Question 9
Why do antibiotics usually fail to control viral infections?
A.
Viruses lack bacterial cell structures and use host cell machinery.
B.
Viruses are killed only by platelets and fibrin threads.
C.
Viruses have cell walls that are impermeable to all chemicals.
D.
Viruses are eukaryotic cells with protected nuclei.
Question 10
What is produced by repeated mitosis of an activated B-lymphocyte?
A.
A clone of cells with the same antibody specificity
B.
A mesh of fibrin threads containing erythrocytes
C.
A population of phagocytes with different antigen receptors
D.
A group of unrelated antibodies with random binding sites
Question 11
Which disease is a zoonosis transmitted to humans mainly by bites from infected mammals?
A.
Childbed fever
B.
Cholera
C.
Rabies
D.
AIDS
Question 12
Outline one physical defence provided by skin.
[1]
Outline one chemical defence provided by skin or mucous membranes.
[1]
State why these are called primary defences.
[1]
Question 13
A small cut in the skin begins to bleed.
State the blood component that releases clotting factors at the cut.
[1]
Outline the role of thrombin in clot formation.
[1]
State one way in which the clot helps defence against disease.
[1]
Question 14
Describe how phagocytes control infection after pathogens enter body tissues. [3]
Question 15
Erythrocyte surface molecules can trigger antibody production after an incompatible blood transfusion.
State the term for a molecule that triggers a specific immune response.
[1]
Explain why an incompatible transfusion can stimulate antibody production.
[1]
Question 16
Outline three mechanisms by which HIV can be transmitted. [3]
Question 17
A student investigated clotting time in blood samples with different platelet concentrations.
Identify the platelet concentration with the shortest clotting time.
[1]
Describe the relationship between platelet concentration and clotting time.
[1]
Explain how platelets help produce a clot after skin is cut.
[1]
Question 18
Phagocytes were observed after exposure to bacteria in a tissue culture.
State the time interval during which the proportion of phagocytes containing bacteria increased most rapidly.
[1]
Describe the change in the proportion of phagocytes containing bacteria over the whole observation period.
[1]
Explain how bacteria enter and are digested inside phagocytes.
[1]
Question 19
What is required for activation of a B-lymphocyte specific to a pathogen antigen?
A.
Exposure to any antibody already present in the blood plasma
B.
Engulfment of any pathogen and digestion by lysosomal enzymes
C.
Interaction with its specific antigen and contact with a helper T-cell activated by the same antigen
D.
Conversion of fibrinogen into fibrin at the infection site
Question 20
Why is the antibody response faster after a second infection by the same pathogen?
A.
Memory lymphocytes specific to the antigen remain after the first response.
B.
Platelets release antibodies directly into the wound.
C.
Antibiotics permanently remain in the blood after the first infection.
D.
The pathogen loses all of its surface antigens after the first infection.
Question 21
For a pathogen with an average of 4 secondary infections per case in a fully susceptible population, what is the estimated herd immunity threshold using H = (1 − 1/R) × 100%?
A.
75%
B.
50%
C.
25%
D.
80%
Question 22
A country reports twice as many confirmed COVID-19 cases as another country. Which additional information is most important for a fairer comparison of incidence?
A.
Population size and testing rate in each country
B.
The official language used in health reports
C.
The number of letters in each country’s name
D.
The name of the national public health agency
Question 23
What is a false-negative result in a diagnostic test for infection?
A.
The test indicates absence of infection when the person is infected.
B.
The test result is confirmed by a second independent test.
C.
The test indicates infection when the person is not infected.
D.
The test is performed with positive and negative controls.
Question 24
How does antibiotic use act as a selection pressure in a bacterial population?
A.
Viruses acquire bacterial ribosomes and become multiresistant.
B.
Phagocytes divide by mitosis to form antibiotic-resistant clones.
C.
All bacteria mutate because the antibiotic needs them to become resistant.
D.
Susceptible bacteria are inhibited while resistant bacteria survive and reproduce.
Question 25
Distinguish between innate and adaptive immunity. [4]
Question 26
Explain why an antibiotic that inhibits bacterial cell wall formation can treat some bacterial infections but not viral infections. [4]
Question 27
A B-lymphocyte binds a pathogen antigen but does not receive contact from a helper T-lymphocyte activated by the same antigen.
State the consequence for full B-cell activation.
[1]
Explain why requiring two matching signals is useful.
[1]
Question 28
Describe how activated B-lymphocytes produce sufficient quantities of a specific antibody during an infection. [4]
Question 29
Compare the primary and secondary antibody responses to the same antigen. [3]
Question 30
Explain why advanced HIV infection can lead to opportunistic infections. [4]
Question 31
Outline zoonoses using tuberculosis, rabies and Japanese encephalitis as examples. [3]
Question 32
A pathogen has R = 5 in a fully susceptible population.
Calculate the herd immunity threshold using H = (1 − 1/R) × 100%.
[1]
State why the threshold is only an estimate.
[1]
Question 33
A public health team recorded newly diagnosed HIV infections by reported exposure category.
| Reported exposure category | New HIV diagnoses / cases |
|---|---|
| Shared needles | 155 |
| Condomless sex | 420 |
| Mother-to-child transmission | 38 |
| Blood products | 12 |
| No reported risk | 75 |
| Total | 700 |
Identify the exposure category associated with the greatest number of diagnoses.
[1]
Calculate the percentage of diagnoses in the category identified in (a).
[1]
Suggest two public health measures that could reduce HIV transmission in the categories shown.
[1]
Question 34
The graph shows antibody concentration after vaccination and after later exposure to the same pathogen antigen.
Identify which response is the secondary response.
[1]
Compare the antibody concentration changes in the two responses.
[1]
Explain how vaccination can lead to immunity without causing the disease.
[1]
Question 35
Discs containing different antibacterial antibiotics were placed on agar inoculated with a bacterial species.
Identify the antibiotic with the greatest effectiveness against the bacteria.
[1]
Describe the trend in the zone of inhibition for the antibiotic tested at increasing concentration.
[1]
Suggest two controlled variables for this investigation.
[1]
Explain why the same antibiotics should not be expected to inhibit a virus grown in human cells.
[1]
Question 36
A hospital bacterial strain becomes resistant to three antibiotics used frequently on a ward.
State the term for bacteria resistant to multiple antibiotics.
[1]
Explain how frequent antibiotic use can increase the frequency of resistant bacteria.
[1]
Question 37
A rapid diagnostic test for a respiratory virus is introduced at an airport.
Define a false-positive result.
[1]
Suggest three ways to reduce misleading diagnostic results.
[1]
Question 38
A bacterial strain was exposed repeatedly to three antibiotics. The minimum inhibitory concentration (MIC) was measured after each round of exposure.
| Antibiotic | Before exposure MIC / mg dm⁻³ | Round 1 MIC / mg dm⁻³ | Round 2 MIC / mg dm⁻³ | Round 3 MIC / mg dm⁻³ |
|---|---|---|---|---|
| Tetracycline | 1.0 | 2.0 | 4.0 | 8.0 |
| Erythromycin | 2.0 | 3.0 | 4.5 | 6.0 |
| Gentamicin | 0.50 | 0.75 | 1.25 | 2.0 |
State which antibiotic shows the greatest increase in MIC.
[1]
Calculate the percentage change in MIC for this antibiotic.
[1]
Explain how the data support the evolution of antibiotic resistance.
[1]
Question 39
The graph shows vaccination coverage and the number of reported cases of a contagious disease over several years in a region.
Describe the relationship between vaccination coverage and reported cases.
[1]
Use the graph to identify a year in which the region was below the estimated herd immunity threshold.
[1]
Explain how herd immunity reduces the risk of an epidemic.
[1]
Suggest one limitation of using these data alone to judge vaccine effectiveness.
[1]
Question 40
COVID-19 data from two regions are shown for the same month.
| Region | Population / people | Confirmed cases / cases | Deaths / deaths | Diagnostic tests / tests | Vaccinated / % |
|---|---|---|---|---|---|
| A | 5,200,000 | 48,000 | 720 | 310,000 | 68 |
| B | 4,900,000 | 36,000 | 360 | 620,000 | 84 |
Calculate deaths as a percentage of confirmed cases for Region A.
[1]
Compare the death percentages in the two regions.
[1]
Evaluate two limitations of using deaths as a percentage of confirmed cases to compare the severity of outbreaks.
[1]
Question 41
A new diagnostic test for a viral infection was compared with a reference laboratory method.
| New test result | Reference infected (people) | Reference not infected (people) |
|---|---|---|
| Positive | 108 | 9 |
| Negative | 12 | 171 |
Identify the number of false-positive results.
[1]
Identify the number of false-negative results.
[1]
Calculate the percentage of infected people correctly detected by the new test.
[1]
Suggest one reason for repeating or confirming important results.
[1]
Question 42
Outline how skin and mucous membranes act as primary defences against pathogens.
[1]
Explain how the innate and adaptive immune systems defend the body after a pathogen enters tissues.
[1]
Question 43
State where lymphocytes are found and what B-lymphocytes produce.
[1]
Explain how antigens lead to the production of a specific antibody.
[1]
Question 44
Outline two ways antibiotics can act selectively against bacteria.
[1]
Discuss why careful antibiotic use is important for individual treatment and population-level resistance.
[1]
Question 45
Define vaccine and immunization.
[1]
Explain how vaccination can protect both an individual and a population.
[1]
Question 46
The graph shows helper T-lymphocyte concentration and incidence of opportunistic infections in untreated HIV infection over time.
Describe the change in helper T-lymphocyte concentration over time.
[1]
State the relationship between helper T-lymphocyte concentration and opportunistic infection incidence.
[1]
Explain why the incidence of opportunistic infections changes as shown.
[1]
Suggest why antibody production may be reduced even though B-lymphocytes are not the main cells infected by HIV.
[1]
Question 47
Outline the two signals required to activate a B-lymphocyte.
[1]
Explain how activation of B-lymphocytes leads to antibody production and long-term immunity.
[1]
Question 48
State two derived values that allow fairer comparisons of COVID-19 data between countries than raw totals.
[1]
Evaluate the use of confirmed case and death data for comparing the impact of COVID-19 between countries.
[1]
Question 49
Define zoonosis and give one named example.
[1]
Discuss why zoonotic diseases are a continuing risk to humans, using examples.
[1]
Question 50
Calculate and interpret the herd immunity threshold for a pathogen with R = 8.
[1]
Evaluate the role of vaccination programmes in preventing epidemics, including benefits and limitations.
[1]
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