IB DP Biology: Defence against disease: immunity, HIV and vaccines (DP Biology) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – the IB holds copyright in its own papers. Use these alongside the official past papers available through your school or the IB store.

These questions cover section C3.2 (Defence against disease) of the IB Biology guide, first assessment 2025, for DP Biology. All content is SL and HL; the guide has no additional higher level content here. All data and places in the questions are fictional.

Read the study guide and revision notes first if you need to. See also the IB DP Biology hub, the printable checklist and the free 10-minute diagnostics.

Questions

1. State three groups of organism that are usually called pathogens, and state whether archaea are known to cause human disease. [3]

2. Explain how skin and mucous membranes help to prevent infection. [3]

3. Outline how a cut in the skin is sealed by blood clotting. [4]

4. Distinguish between the innate and the adaptive immune system. [3]

5. Describe how a phagocyte removes bacteria from an infected wound. [4]

6. Explain why a B-cell needs contact with a helper T-cell as well as binding to its antigen before it can make antibody. [4]

7. One activated B-cell divides every 8 hours, and every cell keeps dividing.

(a) Calculate the number of cells after 72 hours. [2] (b) Calculate the least time needed to reach 5000 cells. [2]

8. In a fictional experiment a volunteer receives the same harmless antigen twice. Peak antibody concentration was 25 arbitrary units after dose one and 800 after dose two.

(a) Calculate the percentage change in peak concentration. [2] (b) Explain the difference between the two responses. [3]

9. (a) State two ways in which HIV can be transmitted. [2] (b) Explain why a person with advanced HIV infection suffers from opportunistic infections. [3]

10. A hospital culture contains 25 bacteria resistant to an antibiotic.

(a) Explain why the antibiotic blocks bacterial growth without harming human cells. [2] (b) Explain why the same antibiotic does not treat a virus infection. [2] (c) The antibiotic kills all non-resistant bacteria. The resistant bacteria divide every 30 minutes. Calculate their number after 5 hours. [2] (d) State one action that slows the spread of multiresistant bacteria. [1]

11. (a) Describe how rabies is transferred to humans and why it is a zoonosis. [2] (b) A vaccine contains messenger RNA coding for a surface protein of a virus. Explain how it can give immunity without causing the disease. [3]

12. Fictional data from the town of Brackenfield during an outbreak of a respiratory virus: weekly cases were 1250 in week 2 and 3500 in week 6. Hospital admissions per 100 000 people were 60 in the unvaccinated group and 15 in the vaccinated group. Of 6000 residents, 4200 were immune. A newspaper claims “vaccination caused the lower admissions”.

(a) Calculate the percentage change in weekly cases from week 2 to week 6. [2] (b) Calculate the percentage difference between the two admission rates. [2] (c) Calculate the percentage change in admissions from unvaccinated to vaccinated. [1] (d) Calculate the percentage of residents who were immune. [1] (e) Evaluate the newspaper’s claim. [4]

Answers

1. Any three of viruses, bacteria, fungi, protists [2]. Archaea are not known to cause any human disease [1]. Examiner insight: A list of three needs three different groups; naming two bacteria is still one group.

2. Skin is a physical barrier: tough keratinised outer layer that microorganisms cannot easily cross [1]. Skin secretions are chemical barriers that discourage microbial growth [1]. Mucus on membranes traps microorganisms, and cilia move it away [1]. Examiner insight: Say which point is physical and which is chemical, because the question asks you to explain how the barrier works.

3. Platelets release clotting factors [1]. A cascade of reactions follows [1]. Thrombin converts soluble fibrinogen to insoluble fibrin [1]. Fibrin traps erythrocytes, forming a clot that seals the cut [1]. Examiner insight: For an outline, the order matters, so write the steps in the order they happen.

4. Innate responds to broad categories of pathogen; adaptive responds to a specific pathogen [1]. Innate does not change during life; adaptive becomes more effective after exposure [1]. Innate has no memory; adaptive builds memory cells [1]. Examiner insight: A distinguish answer states both sides; writing only the adaptive features leaves the comparison unfinished.

5. Phagocyte moves from the blood to the wound by amoeboid movement [1]. It recognises the bacteria as foreign [1]. It engulfs them by endocytosis into a vesicle [1]. Lysosome enzymes digest the bacteria [1]. Examiner insight: Name the process (endocytosis) and the organelle (lysosome); a vague phrase such as “eats the bacteria” is weaker.

6. Activation needs two signals: the specific antigen [1] and a helper T-cell that has been activated by the same type of antigen [1]. Without both, the B-cell is not activated [1], so it neither forms plasma cells that secrete antibody nor memory cells [1]. Examiner insight: Mention that the helper T-cell must respond to the same antigen type, since the link to specificity is the point.

7. (a) 72 ÷ 8 = 9 divisions [1]. 2⁹ = 512 cells [1] (b) 2¹² = 4096 (too few), 2¹³ = 8192, so 13 divisions [1]. 13 × 8 = 104 hours [1] Examiner insight: In a doubling question, round the number of divisions up to a whole number, because a part-division does not exist, and show the working.

8. (a) (800 − 25) ÷ 25 × 100 [1] = 3100% [1] (b) First dose: few matching B-cells, so a slower clone build-up and lower peak [1]. First dose produced memory cells [1]. Second dose: memory cells respond faster, forming many plasma cells and a larger antibody peak [1]. Examiner insight: Quote the data and say “memory cells”, because a bare “the body remembers” does not name the cells responsible.

9. (a) Any two of unprotected sex, shared needles, mother to child, unscreened blood transfusion, breast milk [2] (b) HIV infects and kills helper T-cells [1]. Fewer B-cells are activated, so less antibody is made [1]. Microorganisms that rarely harm a healthy person then cause infections [1]. Examiner insight: Name helper T-cells, not “white blood cells”, and link the loss to antibody production.

10. (a) It blocks a process that bacteria have, such as building a cell wall or bacterial ribosome function [1]. Human cells lack that process or structure [1]. (b) Viruses have no metabolism of their own or ribosomes and use the host cell’s machinery [1], so there is no bacterial process for the antibiotic to block [1]. (c) 5 ÷ 0.5 = 10 divisions [1]. 25 × 2¹⁰ = 25 600 [1] (d) Any one of: finish the prescribed course, use only when needed, avoid routine use in farm animals [1]. Examiner insight: An explanation needs both parts: what the drug targets and why the host or virus lacks that target.

11. (a) Virus passes in saliva through a bite from an infected mammal [1]. It is a zoonosis because the infection transfers from other species to humans [1]. (b) The mRNA enters cells, which make the viral protein [1]. The immune system treats it as an antigen and activates lymphocytes [1]. Memory cells form, but no whole virus is present, so no disease [1]. Examiner insight: Do not say the vaccine contains antibodies; say it supplies an antigen or the code for one.

12. (a) (3500 − 1250) ÷ 1250 × 100 [1] = +180% [1] (b) Difference 45, mean 37.5 [1]; 45 ÷ 37.5 × 100 = 120% [1] (c) (15 − 60) ÷ 60 × 100 = −75% [1] (d) 4200 ÷ 6000 × 100 = 70% [1] (e) Vaccinated people had lower admission rates (60 against 15 per 100 000), which supports a link [1]. Correlation does not show cause: the groups may differ in age or health [1]. The difference in admissions may be partly due to other factors, and the report may precede evaluation by other scientists [1]. Judgement: the data are consistent with a protective effect but do not alone prove the claim [1]. Examiner insight: An evaluation scores best with a limitation, a supporting point and a reasoned judgement, not a simple yes or no.

Where marks are usually lost

• Listing organisms of one group when asked for different groups of pathogen.
• Saying vaccines contain antibodies, or that antibiotics kill viruses.
• Omitting the helper T-cell from B-cell activation.
• Writing “white blood cells” when helper T-cells are needed.
• Not showing the substitution in a percentage change calculation.
• Using the wrong starting value, which changes the sign and size.
• Failing to round a number of divisions up to a whole number.
• Concluding that two trends moving together prove one caused the other.

Next steps

• Review the revision notes and the study guide.
• Go to the IB DP Biology hub and tick off the printable checklist.
• Try all free 10-minute diagnostics.
• Book a free trial class: trial class.

Official syllabus

IB Biology guide, first assessment 2025, published by the International Baccalaureate Organization, section C3.2.