IB Biology SL C3.2 Defence against disease Questions

Practise IB Biology SL C3.2 by explaining blood clotting and immune responses, interpreting infection and vaccine evidence, and tracing antibiotic-resistance patterns.

80 questions in this syllabus scope, with question parts counted together.

Exam analysis

SL · 2010–2025

Papers with this topic68 / 141in the analysed sample

Latest appearanceNovember 2025

Tagged question parts90

Paper types5

Most tested objectives

C3.2.14—Antibiotic resistance evolution17 papers
C3.2.12—HIV infection and AIDS13 papers
C3.2.13—Antibiotics13 papers
C3.2.3—Blood clotting11 papers
C3.2.1—Pathogens cause infectious diseases7 papers

Common question formats

Structured response
Definition or recall
Data analysis
Evaluation
Process explanation
Diagram interpretation
Extended response
Calculation

Recent exam appearances

November 2025Paper 2 SL · TZ3Q1(g)[ 2 ]

November 2025Paper 1A SL · TZ1Q17, 18[ 2 ]

May 2025Paper 1B SL · TZ3Q3(b)(i), 3(b)(ii)[ 2 ]

141 papers analysed · Tagging incomplete

Exam points

Define pathogens and relate named infectious-disease examples to symptoms, transmission, or disease effects.
Trace clotting-factor release and the thrombin conversion of fibrinogen into a fibrin mesh that seals a wound.
Distinguish innate from adaptive immunity and explain how phagocytes detect, engulf, and digest pathogens.
Explain antigen-specific lymphocyte responses, including antibody production, B-cell cloning, and helper-T-cell roles.
Use prior exposure or antibody-response evidence to explain memory-cell protection on reinfection.
Explain how HIV infection damages lymphocytes and reduces antibody production in AIDS.
Explain why antibiotics block bacterial targets but do not treat viral infections.
Use mutation, selection, and resistance data to explain how antibiotic-resistant bacteria survive and spread.
Identify zoonoses and use animal-to-human transmission examples such as rabies or Japanese encephalitis.
Interpret vaccination or antibody-response evidence to distinguish prior immunity and vaccine-induced responses.

Covered syllabus points

C3.2.1—Pathogens cause infectious diseases C3.2.2—Skin and mucous membranes C3.2.3—Blood clotting C3.2.4—Innate vs. adaptive immune system C3.2.5—Phagocytes C3.2.6—Lymphocytes C3.2.7—Antigens C3.2.8—B-lymphocyte activation C3.2.9—Clones of plasma cells C3.2.10—Immunity from memory cells C3.2.11—HIV transmission C3.2.12—HIV infection and AIDS C3.2.13—Antibiotics C3.2.14—Antibiotic resistance evolution C3.2.15—Zoonoses C3.2.16—Vaccines and immunization C3.2.17—Herd immunity C3.2.18—COVID-19 pandemic data evaluation

Exam points

Define a pathogen as a disease-causing organism or virus, and identify bacteria, viruses, fungi, protists or parasites as pathogen types when shown in exam evidence.
Use named pathogen examples such as Vibrio cholerae, Helicobacter pylori, Salmonella, Chlamydia, Streptococcus or Plasmodium to link infection with transmission, toxins, symptoms or disease effects.

[Maximum number: 4]

Cholera is an acute intestinal disease caused by the bacterium Vibrio cholerae. It is transmitted in contaminated drinking water or food.

(a)

Explain how V. cholerae causes diarrhea.

[ 3 ]

a. (the bacterium) produces a toxin;

b. toxin enters/binds to intestinal/epithelial cells;

c. causes ions ( $\mathrm{Cl}^{-}$ and $\mathrm{HCO}_{3}^{-}$ ) to move into lumen of intestine;

d. water leaves the cells by osmosis;

e. causing watery feces/diarrhea;

e. Do not accept diarrhea alone

- watery or similar required.

(b)

State how cholera can cause death.

[ 1 ]

Exam points

Explain how keratinized skin and shedding form a physical barrier that limits pathogen entry.
Explain how mucus and cilia trap or remove pathogens, while lysozyme and acidic gastric juice damage or destroy them on respiratory and digestive mucous membranes.

[Maximum number: 7]

According to the cell theory, living organisms are composed of cells.

Explain the role of cells in the defence against infectious disease.

Exam points

Explain that platelets and damaged tissue release clotting factors that trigger a cascade producing or activating thrombin at a wound.
Explain that thrombin converts soluble fibrinogen to insoluble fibrin; the fibrin mesh traps blood cells and seals the wound or entry point.

[Maximum number: 1]

A cut in the skin triggers a cascade of reactions controlled by several blood components that results in the quick formation of a clot.

In which order do these blood components act to form a blood clot?

platelets - clotting factors - thrombin - fibrinogen - fibrin

platelets - fibrinogen - fibrin - thrombin - clotting factors

clotting factors - platelets - thrombin - fibrinogen - fibrin

clotting factors - platelets - thrombin - fibrin - fibrinogen

Exam points

Distinguish innate immunity as a broad, rapid defence present from birth that does not become more specific, from adaptive immunity as an acquired response targeted to particular antigens or pathogens.
Use immune-response evidence to identify adaptive specificity and memory in lymphocyte or antibody responses, contrasted with non-specific phagocyte and barrier defences.

[Maximum number: 1]

The body has developed various methods to fight against infectious disease.

I. Fibrinogen is converted to fibrin, which prevents the entry of bacteria and loss of blood.
II. Phagocytic white blood cells recognize pathogens and ingest and destroy them.
III. Lymphocytes produce antibodies in response to pathogens in the blood.

Which provides specific immunity to disease?

I and II only

II and III only

III only

I, II and III

Exam points

Explain how phagocytes use chemotaxis to leave the bloodstream and reach infected tissues where pathogens or foreign material are detected.
Explain phagocytosis as membrane engulfment into a vacuole followed by lysosomal-enzyme digestion and destruction of the pathogen as a non-specific defence.

[Maximum number: 4]

Describe how phagocytic leucocytes may act as a defence against disease.

Exam points

Explain that antigen-specific lymphocytes coordinate adaptive defence: helper T-cells activate selected B-cells, which form plasma cells that secrete antibodies, while T-cells can activate or destroy infected cells.
Explain clonal expansion by mitosis and the formation of memory B- or T-cells, which persist after the primary response and enable faster secondary immunity.

[Maximum number: 1]

Which function is primarily associated with helper T-cells in the immune system?

Phagocytosis of pathogens

Activation of other cells in the immune system

Antibody production

Retention as memory cells

Exam points

Define an antigen as a non-self molecule, often a protein or glycoprotein, whose specific shape is recognized by an antibody or lymphocyte receptor and triggers a specific immune response.
Use Rh antigen exposure as evidence that a first encounter can stimulate specific antibody production and a later immune response against the same antigen.

[Maximum number: 1]

What is a possible explanation for why subsequent pregnancies could trigger an immune response?

Exposure to the $\mathrm{Rh}^{+}$ antigen in the first pregnancy triggered the development of antibodies that could attack the blood of a future $\mathrm{Rh}^{+}$ baby.

Exposure to the $\mathrm{Rh}^{+}$ antigen in the first pregnancy triggered the development of specific phagocytes that could attack the blood of a future $\mathrm{Rh}^{+}$ baby.

The mother has developed passive immunity to the $\mathrm{Rh}^{+}$ factor.

The mother's immune system has been weakened by pregnancy.

Exam points

Explain that an antigen binds to a specific B-cell and that an activated helper T-cell provides the signal that activates the B-cell.
Explain that an activated B-cell enters clonal expansion and differentiates into plasma cells that secrete antibodies specific to the presented antigen.

[Maximum number: 7]

Tuberculosis (TB) is an infectious disease caused by the bacterium Mycobacterium tuberculosis.

Explain the production of antibodies when a patient is infected with the TB bacterium.

Exam points

Explain clonal selection as antigen-specific activation followed by mitotic division of the selected B-cell to produce genetically identical clones.
Explain that B-cell clones differentiate into plasma cells with abundant rough endoplasmic reticulum, which secrete large amounts of one specific antibody.

[Maximum number: 6]

Describe the production of antibodies.

[Maximum number: 1]

A person was infected with a pathogen and then later in life they were re-infected with the same pathogen. The graph shows the concentration of antibodies found in the blood that were produced in response to these two infections.

Time / days
What is the reason for the faster rise in antibody concentration after the second infection?

The patient had previously been vaccinated with the antibody.

Antibodies had been stored after the first exposure.

Lymphocytes rapidly reproduced to form plasma cells.

The second exposure was more infectious.

[Maximum number: 8]

Discuss the transmission and social implications of AIDS.

Exam points

Explain how HIV infection reduces T-lymphocyte numbers and weakens antibody production.
Explain how helper T-cell loss weakens immune defence and can lead to opportunistic infections as AIDS develops.

[Maximum number: 1]

What is a method of transmission of HIV and its effect on the immune system?

Method of transmission

Effect on the immune system

Breastfeeding

Decrease in number of active lymphocytes

Blood transfusion

Overall, a general increase in antibody production

Waterborne

Overall, a general reduction in antibody production

Mosquito bites

Increase in number of active lymphocytes

Exam points

Explain that antibiotics selectively block bacterial metabolic or cellular processes that are absent from eukaryotic cells.
Explain why antibiotics do not treat viral disease: viruses lack the bacterial processes that antibiotics target.
Distinguish antibiotics from antivirals: antibiotics do not treat viral infections, whereas antiviral drugs are used against viral pathogens.

[Maximum number: 1]

What property of antibiotics makes them effective in the treatment of infectious diseases?

They stimulate the production of antibodies.

They block metabolic pathways in prokaryotes.

They block the metabolic processes in viruses.

They inhibit mitosis in eukaryotes.

Exam points

Explain how mutations or plasmids can introduce heritable antibiotic resistance into a bacterial population.
Explain how antibiotic use, overuse or incomplete courses select for resistance: susceptible bacteria die while resistant variants survive, reproduce and become more common.
Interpret resistance data by comparing strains and identifying resistance to multiple antibiotics.

[Maximum number: 1]

Azithromycin is an antibiotic commonly used to treat pneumonia, an infection of the lungs caused by Streptococcus pneumoniae. A patient was diagnosed for the second time with pneumonia, but the same antibiotic was ineffective this time.

What biological explanation might there be for this?

S. pneumoniae had mutated.

S. pneumoniae had developed antibodies for the antibiotic.

The patient was immune to the antibiotic.

The patient had antibiotic remaining from the first treatment.

Exam points

Recognize zoonoses as infectious diseases transferred from animals to humans.
Identify rabies, tuberculosis and Japanese encephalitis as examples of zoonoses.

[Maximum number: 1]

What do rabies, tuberculosis and Japanese encephalitis have in common?

They are all infectious diseases caused by bacteria.

They are all infectious diseases caused by viruses.

They are all zoonoses.

They are all infecting red blood cells.

[Maximum number: 2]

The image shows a castor bean tick (Ixodes ricinus) which is commonly found in woodlands or forest areas. This tick can carry bacteria that cause Lyme disease in humans.

(a)

A vaccine has been developed to protect people against the symptoms of Lyme disease.
The graph shows the relative number of antibodies specific to these bacteria in the blood of a person before vaccination.

[ 2 ]

(i)

Suggest what can be concluded from the graph about the person before they were vaccinated.

[ 1 ]

(ii)

Sketch a line on the graph to show how the number of antibodies would change over time after vaccination.

[ 1 ]

No public sample questions are available for this syllabus point yet.

a	phagocytes/lymphocytes are white blood cells;
b	TB bacterium has a specific antigen;
c	this antigen is recognised by white blood cells;
d	a clone of lymphocytes/plasma cells/B cells are produced;
e	antibodies are produced by lymphocytes;
f	each lymphocyte produces just one type of antibody;
g	(this is) specific immunity;
h	(part of the) antibody/immunoglobulin binds to the antigen / specific antibody binds to the specific antigen;
i	antibodies are proteins/immunoglobulins;
j	some activated B lymphocytes differentiate into memory B cells;
k	memory cells reproduce quickly;
l	memory cells prevent infection in the future;

a	(person had been) previously infected / have immunity / have memory cells (to the bacterium);
b	had been bitten by an (infected) tick before;
c	their immunity is diminishing/reducing;

Exam analysis

Most tested objectives

Common question formats

Recent exam appearances

Covered syllabus points

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