OCR A Level Biology A: Specific immunity, vaccination and medicines (H420) – 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 – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover learning outcomes 4.1.1(f) to 4.1.1(n) of the OCR A Level Biology A (H420) specification, Version 4.1 (April 2026), for first assessment in 2025. Learn the content in the study guide and revision notes first. The course hub, the checklist and the free diagnostics show the wider course.

Questions

1. State one role of each of the following in the specific immune response: T helper cell, T killer cell and plasma cell. [3]

2. Explain what is meant by clonal selection and clonal expansion. [4]

3. A rabbit is injected with an antigen on day 0 and the same antigen again on day 40. The table shows antibody concentration in arbitrary units (AU).

Event	Result
First exposure	Detected on day 6; peak of 18 AU on day 16
Second exposure	10 AU on day 41; peak of 400 AU on day 45

(a) Calculate the average speed at which the concentration rose from day 41 to day 45. [2] (b) Calculate how many times higher the second peak is than the first, to 3 significant figures. [1] (c) Explain the differences between the two responses. [2]

4. Describe the structure of an antibody molecule and explain how its structure allows it to bind to one type of antigen only. [4]

5. Outline the action of (a) opsonins, (b) agglutinins and (c) anti-toxins. [3]

6. Name the type of immunity (for example natural passive) in each situation. (a) A person recovers from measles and does not catch it again. [1] (b) A baby receives antibodies in breast milk. [1] (c) A patient is given antibodies against rabies after a bite. [1] (d) A child is given a vaccine containing a harmless form of a toxin. [1]

7. Explain what is meant by an autoimmune disease, using arthritis as an example. [3]

8. In a trial of a new vaccine, 12 000 volunteers received the vaccine and 12 000 received a placebo. Over the study period, 15 vaccinated and 135 placebo volunteers developed the disease. (a) Calculate the percentage reduction in the proportion of people developing the disease that is linked to vaccination. [3] (b) The disease has R₀ = 3. The simple model for herd immunity gives the proportion that must be immune as 1 - 1/R₀. Calculate the percentage of the population that must be vaccinated if the vaccine protects 80 % of those vaccinated. [3]

9. Suggest why vaccination programmes change over time, and explain why some changes are needed on a global scale. [5]

10. Explain why the maintenance of biodiversity is relevant to the discovery of medicines, and outline how personalised medicine could improve treatment. [4]

11. A bacterial culture contains 5.0 x 10⁸ cells. One cell in every 10⁷ is resistant to an antibiotic. (a) Calculate how many resistant cells are present. [1] (b) The antibiotic kills every sensitive cell. The resistant cells divide every 20 minutes and none die. Calculate the number of resistant cells after 6 hours, to 3 significant figures. [2] (c) Explain how antibiotic resistance arises and spreads in a bacterial population, and give two implications for the treatment of infection. [5]

Answers

1. T helper: secretes cytokines (interleukins), signalling to B cells and other cells [1]. T killer: destroys body cells that display the antigen, such as virus-infected cells [1]. Plasma cell: secretes a specific antibody [1]. [3] Examiner insight: Each cell needs a role of its own. Answers that say “fights infection” for all three do not show what each cell does, so state a role for each.

2. Many lymphocytes exist, each with a receptor of one shape [1]. Clonal selection: only the lymphocyte whose receptor is complementary to the antigen is activated [1]. Clonal expansion: the selected cell divides by mitosis [1] to form a clone of identical cells with the same receptor [1]. [4] Examiner insight: Name the type of division, and state that the receptor must be complementary. “The cell is chosen” alone is too vague.

3. (a) Rate = (400 - 10) / (45 - 41) [1] = 390 / 4 = 97.5 AU per day [1]. (b) 400 / 18 = 22.2 times [1]. (c) The first exposure produced B memory cells (and T memory cells) [1], which are many and already selected, so the second response is faster and larger with no delay to find a matching cell [1]. [5] Examiner insight: A rate needs units that include time. Use two points from the steepest part of the curve and show the subtraction, so your method is clear.

4. Globulin protein with four polypeptide chains [1]: two heavy and two light, held by disulfide bridges [1]. The variable region at the tip of each arm has a particular amino acid sequence and tertiary shape [1] that is complementary to one antigen, like an enzyme and substrate, so other antigens do not fit [1]. [4] Examiner insight: Link the specificity to the variable region, not the whole molecule. Naming the hinge or constant region does not explain binding.

5. (a) Opsonins bind to the pathogen so phagocytes engulf it more readily [1]. (b) Agglutinins cause pathogens to clump together, making them easier to engulf [1]. (c) Anti-toxins bind to and neutralise toxins [1]. [3] Examiner insight: “Outline” needs a clear action and, where possible, its effect. Repeating the term from the question does not describe an action.

6. (a) Natural active [1]. (b) Natural passive [1]. (c) Artificial passive [1]. (d) Artificial active [1]. [4] Examiner insight: Decide first whether the person made their own antibodies (active) or was given them (passive), then whether it was a medical procedure. Two words are needed.

7. An autoimmune disease occurs when the immune system fails to distinguish self from non-self [1] and attacks the body’s own tissues [1]. In rheumatoid arthritis the joints are attacked, causing inflammation, pain and damage to cartilage [1]. [3] Examiner insight: Say “own cells or tissues”, not “attacks the body”, and name the tissue affected in the example.

8. (a) Vaccinated: 15 / 12 000 = 0.125 % [1]. Placebo: 135 / 12 000 = 1.125 % [1]. Reduction = (1.125 - 0.125) / 1.125 x 100 = 88.9 % [1]. (b) Immune needed = 1 - 1/3 = 0.667 (66.7 %) [1]. Coverage = 66.7 / 0.80 [1] = 83.3 % [1]. [6] Examiner insight: Compare proportions, not raw case numbers, although here the group sizes are equal. Give the intermediate values so your method is clear.

9. Any five from: pathogens change, such as influenza surface antigens varying, so the vaccine must be updated [1]; new diseases appear and need new vaccines [1]; a disease may be eradicated, so vaccination can stop [1]; coverage may fall and outbreaks follow, so programmes are revised [1]; global issues such as cost, storage and transport, conflict or travel spreading disease across borders make coordinated action necessary [1]. [5] Examiner insight: Pair each change with its reason. A list of vaccines without reasons does not answer “why”.

10. Species not yet studied may contain useful chemicals [1], so loss of biodiversity through extinction may lose potential medicines [1]. Personalised medicine uses a person’s genome to choose the drug or dose [1], giving a better effect with fewer side effects [1]. [4] Examiner insight: Give an actual link, not “biodiversity is important”. Name what is lost and why that matters for medicines.

11. (a) 5.0 x 10⁸ / 10⁷ = 50 [1]. (b) 6 h = 18 doublings [1]. 50 x 2¹⁸ = 1.31 x 10⁷ [1]. (c) Mutation occurs by chance in a gene before exposure [1]. The antibiotic acts as a selection pressure and kills sensitive cells [1]. Resistant cells survive, reproduce and pass on the allele, including to other bacteria on plasmids [1]. Implications, any two: infections harder to treat, longer illness, cost or need for new antibiotics [2]. [8] Examiner insight: The key idea is selection, not adaptation. Bacteria do not change because of the antibiotic, so avoid wording that suggests they do.

Where marks are usually lost

• Describing a rate without units that include time.
• Writing that antibodies are complementary to the pathogen rather than to the antigen.
• Saying T cells produce antibodies.
• Naming only one word for the type of immunity when two are needed.
• Describing herd immunity without saying it protects people who are not immune.
• Treating antibiotic resistance as bacteria “getting used to” the drug.
• Listing risks of antibiotics without describing resistance in terms of mutation and selection.
• Using raw numbers when proportions or percentages are needed.

Next steps

• Revisit the revision notes and study guide.
• Open the course hub and the checklist.
• Try all free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

OCR, A Level Biology A (H420) specification, Version 4.1 (April 2026), first assessment 2025: learning outcomes 4.1.1(f) to 4.1.1(n).