IB Biology · Theme C: Interaction and interdependence · SL and HL
C3.2 Defence against disease
A one-page summary of C3.2 Defence against disease, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
How do body systems recognize pathogens and fight infections?
What factors influence the incidence of disease in populations?
What C3.2 covers
First-line and innate defence
- C3.2.1Not every microbe is an enemy
- C3.2.1 · NOSCareful observation, before germ theory
- C3.2.2Skin blocks two different ways
- C3.2.2Where skin can't seal the gap
- C3.2.3Sealing a cut, step by step
- C3.2.4Two systems, two strategies
- C3.2.5A cell that hunts by crawling
Adaptive immunity — the antibody response
- C3.2.6One cell, one antibody
- C3.2.7The tag that says foreign
- C3.2.8Two signals, not one
- C3.2.9From a few cells to an army
- C3.2.10A response that remembers
HIV, AIDS and antibiotics
- C3.2.11One virus, specific routes
- C3.2.12A virus that targets the helpers
- C3.2.13Why antibiotics can't touch a virus
- C3.2.14Resistance evolves, it isn't learned
Public health — zoonoses, vaccines and herd immunity
- C3.2.15When a disease crosses over
- C3.2.16Training the immune system safely
- C3.2.17Protection that spreads beyond the individual
- C3.2.17 · NOSPublishing is not the final word
- C3.2.18 · SkillsTwo formulas, two different questions
C3.2 Defence against disease: summary
Innate defence
- Skin/mucous membranes (physical + chemical) and blood clotting act before any immune cell is involved.
- Phagocytes engulf pathogens by endocytosis and digest them with lysosomal enzymes — broad, unchanging, no memory.
Innate vs adaptive
- Innate: broad, fast, unchanging. Adaptive: specific, slower, builds memory and improves with exposure.
- This course requires no innate component beyond phagocytes.
Antibody response
- B-cell activation needs two signals — antigen contact + a matching activated helper T-cell.
- Activated B-cells divide by mitosis into plasma cells (antibody factories) and memory cells (long-term immunity).
Primary vs secondary response
- Primary: slow, low peak, first exposure. Secondary: fast, high peak, driven by memory cells.
- Vaccines deliberately trigger this same pathway without causing the disease.
HIV, AIDS & antibiotics
- HIV destroys helper T-cells specifically, collapsing antibody responses and causing AIDS.
- Antibiotics block bacteria-specific processes — useless against viruses; resistance spreads by natural selection.
Public health
- Zoonoses (tuberculosis, rabies, Japanese encephalitis, COVID-19) transfer from animals to humans.
- Herd immunity lowers population-wide exposure; use percentage change (before/after) vs difference (independent values) correctly.
Key terms
- Pathogen
- A disease-causing organism — typically a virus, bacterium, fungus or protist.
- Antigen
- A recognition molecule, usually a surface protein, that the adaptive immune system identifies as foreign.
- Phagocyte
- An innate immune cell that engulfs and digests pathogens by endocytosis, without specificity for one pathogen type.
- Memory cell
- A long-lived lymphocyte, formed after activation, responsible for a faster secondary immune response.
- Herd immunity
- Population-level protection that occurs when enough individuals are immune to impede a pathogen's transmission.
- Zoonosis
- An infectious disease that can transfer from another animal species into humans.
- Vaccine
- A preparation containing antigens, or instructions to make them, that stimulates immunity without causing disease.
- Antibiotic resistance
- The spread, by natural selection, of bacterial traits that survive antibiotic exposure.
- AIDS
- The immune collapse that follows HIV's destruction of helper T-lymphocytes.
Sample exam questions
Three of the 54 multiple-choice questions for C3.2. Try each one before opening the answer.
Question 1. Which group of organisms is NOT typically included when biologists use the term "pathogen"?
- Bacteria
- Archaea
- Fungi
- Protists
Show the answer
Answer: B. The term pathogen is typically reserved for viruses, bacteria, fungi and protists. Archaea are not known to cause disease in humans, despite being a whole separate domain of life.
Question 2. Ignaz Semmelweis reduced deaths from childbed fever on a Vienna maternity ward in the 1840s by requiring doctors to:
- Isolate all patients showing a fever
- Boil all surgical instruments before use
- Vaccinate mothers against streptococcal infection
- Wash their hands with a chlorinated solution between the autopsy room and the maternity ward
Show the answer
Answer: D. Semmelweis noticed higher mortality on the ward attended by doctors coming straight from autopsies. Requiring handwashing with a chlorinated solution sharply reduced deaths — decades before germ theory was formally established, purely from careful observation.
Question 3. A B-cell binds its matching antigen but never encounters a matching activated helper T-cell. What is the most likely outcome?
- The B-cell still fully activates and produces antibodies
- The B-cell fails to fully activate and does not proceed to divide into plasma/memory cells
- The B-cell becomes a phagocyte instead
- The B-cell activates a different, unrelated B-cell clone
Show the answer
Answer: B. Since both antigen contact and helper T-cell contact are required for activation, antigen contact alone leaves the B-cell unactivated — it will not proceed to clonal division and differentiation.
Linking questions
Questions that connect C3.2 to other parts of the course, the kind that come up in Paper 2.
- Antibodies are proteins with a variable region that determines their antigen specificity (B1.2). Explain how the structure of an antibody enables it to bind one specific antigen shape. (see B1.2)
- Phagocytes engulf pathogens by endocytosis, a process that depends on the fluid, flexible nature of the plasma membrane (B2.1). Explain why the fluid mosaic structure of the membrane is necessary for endocytosis to occur. (see B2.1)
- Antibiotic resistance in bacteria (C3.2.14) is an example of natural selection acting on a population (A4.1). Explain how the process of natural selection accounts for the spread of resistance through a bacterial population over time. (see A4.1)
Practise C3.2
Study notes, every question and full markschemes for C3.2 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.