Biology  by Bradford
support
IB Biology 2025 · Theme C · Interaction and interdependence

C3.2 Defence against disease. Practice questions with markscheme.

65 original IB-style questions on C3.2, written from the 2025 guide: 31 multiple-choice, 24 short-answer, 5 data-based, 3 extended-response parts, 1 drawing, 1 labelling. This subtopic has no additional higher level content, so the 20-mark practice paper below serves SL and HL alike. Hand one out as a class quiz or homework, or sit it yourself and mark against the scheme. Print it, project it, or build a fresh one on the same topic.

What the guide asks for

18 statements at SL and HL.

  1. C3.2.1SL / HL Pathogens as the cause of infectious diseases
  2. C3.2.2SL / HL Skin and mucous membranes as a primary defence
  3. C3.2.3SL / HL Sealing of cuts in skin by blood clotting
  4. C3.2.4SL / HL Differences between the innate immune system and the adaptive immune system
  5. C3.2.5SL / HL Infection control by phagocytes
  6. C3.2.6SL / HL Lymphocytes as cells in the adaptive immune system that cooperate to produce antibodies
  7. C3.2.7SL / HL Antigens as recognition molecules that trigger antibody production
  8. C3.2.8SL / HL Activation of B-lymphocytes by helper T-lymphocytes
  9. C3.2.9SL / HL Multiplication of activated B-lymphocytes to form clones of antibody-secreting plasma cells
  10. C3.2.10SL / HL Immunity as a consequence of retaining memory cells
  11. C3.2.11SL / HL Transmission of HIV in body fluids
  12. C3.2.12SL / HL Infection of lymphocytes by HIV with AIDS as a consequence
  13. C3.2.13SL / HL Antibiotics as chemicals that block processes occurring in bacteria but not in eukaryotic cells
  14. C3.2.14SL / HL Evolution of resistance to several antibiotics in strains of pathogenic bacteria
  15. C3.2.15SL / HL Zoonoses as infectious diseases that can transfer from other species to humans
  16. C3.2.16SL / HL Vaccines and immunization
  17. C3.2.17SL / HL Herd immunity and the prevention of epidemics
  18. C3.2.18SL / HL Evaluation of data related to the COVID-19 pandemic

In the bank for C3.2

  • 31 multiple-choice
  • 24 short-answer
  • 5 data-based
  • 3 extended-response parts
  • 1 drawing
  • 1 labelling
  • 0 higher level only

Every question is original and tagged to a guide statement. See the whole bank →

Make your own

The practice paper

No additional higher level content in this subtopic, so one paper serves both · 20 marks
Take it online → Build a fresh paper Paper code BbB-EAAAAAACABQAOz_g
Biology · topic quiz
Standard level · topic practice, not an exam format
30 minutes20 marks

Paper code: this paper was generated, so the code is its recipe. Enter it at biologybybradford.com/exam-maker to rebuild this exact paper and its markscheme.

Covers C3.2 Defence against disease · SL
Name:
1.

A patient receives a transfusion of erythrocytes from a donor of a different blood group. The figure shows the glycoproteins on the surface of the donor's erythrocytes and of the patient's own erythrocytes. Which response is expected in the patient?

[1]
donor erythrocyte patient's own erythrocyte glycoprotein P glycoprotein Q
  1. Antibodies are produced against both glycoprotein P and glycoprotein Q
  2. Antibodies are produced against glycoprotein P, because the patient's cells also carry it
  3. No antibodies are produced, because erythrocytes are not pathogens
  4. Antibodies are produced against glycoprotein Q but not against glycoprotein P
2.

When a high enough proportion of a population is vaccinated, even unvaccinated individuals are protected. Why?

[1]
  1. The vaccine itself is passed from person to person by contact
  2. Antibodies from vaccinated people spread to unvaccinated people
  3. Unvaccinated people develop immunity by contact with vaccinated people
  4. With few susceptible hosts, chains of transmission are broken
3.

Which structures form the body's first line of defence against pathogens?

  • I. The skin as a physical (and chemical) barrier
  • II. Mucous membranes lining airways and other openings
  • III. Antibodies in the blood
[1]
  1. I and II only
  2. I and III only
  3. II and III only
  4. I, II and III
4.

B-lymphocytes specific to antigen X were taken from mice (Mus musculus) and cultured under four conditions. After five days, the concentration of antibody against antigen X in each culture was measured. Which conclusion is supported by the results?

[1]
CultureAntigen X addedHelper T-lymphocytes addedAntibody against antigen X / arbitrary units
1yesnone0.2
2noactivated by antigen X0.3
3yesactivated by antigen X9.6
4yesactivated by antigen Y0.2
  1. Helper T-lymphocytes activated by antigen X can activate these B-lymphocytes without antigen X
  2. Any activated helper T-lymphocyte can activate a B-lymphocyte that has bound antigen X
  3. The B-lymphocytes need both antigen X and helper T-lymphocytes activated by antigen X
  4. Antigen X alone is enough to activate B-lymphocytes specific to antigen X
5.

Electron micrographs show that phagocytes contain many more lysosomes than most other body cells. How does this help phagocytes to control an infection?

[1]
  1. Lysosomes carry receptors that recognize the specific antigens of each type of pathogen
  2. Lysosomes fuse with vesicles containing engulfed pathogens and digest them with enzymes
  3. Lysosomes supply the ATP needed for amoeboid movement to the site of infection
  4. Lysosomes release antibodies that bind to and label pathogens in the tissue fluid
6.

In a maternity hospital in the 1840s, many mothers died of childbed fever, an infection that develops after giving birth. Clinic 1 was staffed by doctors and medical students, who often came straight from dissecting dead bodies. Clinic 2 was staffed by midwives, who did no dissections. The graph shows the percentage of mothers who died in each clinic. Which conclusion about the cause of the fever is best supported by the data?

[1]
0 2 4 6 8 10 12 14 16 1841 1842 1843 1844 1845 1846 1847 1848 hand-washing in chlorine solution introduced in clinic 1 (May 1847) mothers who died / % year clinic 1 clinic 2
  1. The fever was caused by an inherited weakness that was more common in the mothers admitted to clinic 1
  2. The fever was caused by something carried on the hands of the doctors and students from the dead bodies
  3. The fever was caused by the stress to mothers of being examined by doctors rather than by midwives
  4. The fever was spread through the air of the hospital and reached the wards of clinic 1 before clinic 2
7.

When a B-lymphocyte is activated during an infection, it divides repeatedly. Why is this division necessary?

[1]
  1. To replace phagocytes destroyed by the pathogen
  2. To create B cells with many different antibody types
  3. To form a large clone of antibody-secreting plasma cells
  4. To repair the tissues damaged by the pathogen
8.

HIV gradually destroys helper T-lymphocytes. Why does this lead to AIDS?

[1]
  1. Helper T-cells are needed to activate B-cells and other immune cells
  2. The virus turns infected lymphocytes into new pathogens
  3. Helper T-cells themselves secrete all of the antibodies found in blood plasma
  4. Helper T-cells engulf and digest most pathogens themselves by phagocytosis
9.

In one city there were 250 confirmed COVID-19 cases in week 1 and 350 confirmed cases in week 2. What is the percentage change in weekly cases?

[1]
0 50 100 150 200 250 300 350 400 250 week 1 350 week 2 confirmed COVID-19 cases per week one city, two consecutive weeks
  1. +40.0 %
  2. +71.4 %
  3. +28.6 %
  4. −28.6 %
10.

Two groups of mice (Mus musculus) were infected with a species of bacterium that causes a mild illness. One group was a normal strain. The other group was a strain that cannot produce lymphocytes but has normal phagocytes. Eight weeks later, when all the mice had recovered, both groups were infected again with the same bacterium. Three days after each infection, the number of bacteria in the spleen of each mouse was counted. The table shows the mean results.

Strain of miceMean bacteria in spleen 3 days after first infectionMean bacteria in spleen 3 days after second infection
normal4.0 × 10⁵2.0 × 10³
unable to produce lymphocytes5.0 × 10⁵5.2 × 10⁵
(a)Calculate how many times fewer bacteria were present in the normal mice after the second infection than after the first infection.[1]
(b)Suggest why the numbers of bacteria after the first infection were similar in the two strains.[2]
(c)Explain the difference between the two strains after the second infection.[3]
11.

A person has a bacterial infection in a cut on one hand. Tissue fluid from the hand drains into lymph vessels, and this lymph passes through lymph nodes in the armpit. A few days later, the lymph nodes in the armpit on the same side of the body are swollen, but the person's other lymph nodes are not.

(a)State one place in the body, other than lymph nodes and lymph vessels, where lymphocytes are found.[1]
(b)Suggest why only the lymph nodes in that armpit become swollen.[2]
(c)Lymphocytes do not stay in one lymph node: they continually leave the lymph nodes, circulate around the body and enter lymph nodes again. Suggest how this movement helps the adaptive immune system to respond to an infection wherever it occurs in the body.[2]

Original practice questions © Biology by Bradford · CC BY-NC-SA 4.0 · Not affiliated with or endorsed by the International Baccalaureate Organization.
Rebuild or edit this exact paper (and its markscheme): biologybybradford.com/exam-maker?code=BbB-EAAAAAACABQAOz_g

Show the markscheme

Markscheme BbB-EAAAAAACABQAOz_g

One mark per point; / separates alternative wording within a point, OR separates alternative answers, words in brackets are not required, underlined words are essential. OWTTE = or words to that effect.

1. C3.2.7 [1]
  • D: glycoproteins on the surface of erythrocytes act as antigens when they are not found on the recipient's own cells; P is present on the patient's cells, so only Q is foreign and stimulates antibody production; antigens need not belong to pathogens, and molecules the patient's own cells carry are recognized as self, so they do not trigger antibody production;
2. C3.2.17 [1]
  • D: herd immunity: transmission fizzles out when most contacts are immune, indirectly shielding those who cannot be vaccinated;
3. C3.2.2 [1]
  • A: skin and mucous membranes block entry; antibodies belong to the adaptive response after infection;
4. C3.2.8 [1]
  • C: antibody was produced only in culture 3; culture 1 (antigen but no helper T-cells) rules out A, culture 4 (helper T-cells activated by a different antigen) rules out B, and culture 2 (no antigen) rules out C; activation needs both binding of the specific antigen and contact with a helper T-cell activated by the same antigen;
5. C3.2.5 [1]
  • B: pathogens engulfed by endocytosis end up in vesicles, and lysosomes fuse with these, releasing hydrolytic enzymes that digest the pathogen; antibodies come from plasma cells, ATP from mitochondria, and phagocytes are not specific to particular antigens;
6. C3.2.1 [1]
  • B: deaths were far higher only in the clinic whose staff came from dissections, and fell to the level of clinic 2 once staff washed their hands in chlorine solution; hand-washing changes neither inheritance, stress nor the hospital air, so the cause was carried on the hands (a pathogen, although none had yet been identified); careful observation led to the breakthrough;
7. C3.2.9 [1]
  • C: mitosis of the activated B cell produces a clone of (genetically identical) plasma cells, providing enough cells to secrete the specific antibody in large quantities;
8. C3.2.12 [1]
  • A: without helper-T signalling, antibody production and cell-mediated responses collapse; opportunistic infections and cancers follow;
9. C3.2.18 [1]
  • A: percentage change = (350 − 250) ÷ 250 × 100 = +40.0 % (dividing by the original value, not the final value);
10. C3.2.4
  • (a) [1]
    • 200 (times fewer);

    4.0 × 10⁵ ÷ 2.0 × 10³ = 200.

  • (b) [2 max]
    • three days into a first infection, the bacteria in both strains were being controlled by the innate system/phagocytes (which both strains have);
    • the adaptive response to a first infection takes several days / had not yet had an effect in the normal mice;
    • (because) the few lymphocytes specific to the bacterium must first be activated and multiply before much antibody is made, OWTTE;
  • (c) [3 max]
    • the normal mice produced memory cells specific to the bacterium's antigens during the first infection;
    • on re-infection the memory cells gave a faster and stronger response, so the bacteria were destroyed before they could multiply, OWTTE;
    • (so) the adaptive system builds up a memory of pathogens encountered and becomes more effective;
    • the mice without lymphocytes relied on the innate system/phagocytes alone;
    • the innate system responds to broad categories of pathogen in the same way each time / does not change during life, so the second infection was controlled no better than the first;

    Both strains must be addressed for full marks.

11. C3.2.6
  • (a) [1]
    • (circulating) in the blood;

    Accept spleen, bone marrow, thymus or tonsils.

  • (b) [2 max]
    • lymph from the infected hand carries the bacteria/their antigens to the nearest lymph nodes;
    • lymphocytes specific to these antigens are activated in these nodes;
    • activated lymphocytes divide / form clones, so the number of cells in the node increases and it swells, OWTTE;
    • lymph nodes elsewhere receive no antigen from this infection, so no lymphocytes are activated there;
  • (c) [2 max]
    • only a few lymphocytes are specific to any one antigen, and they are spread thinly through the body;
    • circulation brings each lymphocyte through many lymph nodes, so the rare specific ones are likely to meet their antigen in the node that drains the infected tissue, OWTTE;
    • antigens from infected tissues are collected in lymph nodes, so lymphocytes do not need to search every tissue;
    • after activation, lymphocytes/antibodies/memory cells are carried by the blood to the whole body, OWTTE;

More in Theme C · Interaction and interdependence

All 40 IB Biology subtopics →  ·  Command terms →  ·  AP Biology units →  ·  Open the exam maker →