IB DP Biology: Defence against disease: immunity, HIV and vaccines (DP Biology) – Revision Notes

These revision notes condense section C3.2 (Defence against disease) of the IB Biology guide, first assessment 2025, for DP Biology. Statements C3.2.1 to C3.2.18 are SL and HL, and the guide has no additional higher level content here. For full explanations and worked examples, read the study guide first.

Then test yourself with the practice set. Other links: the IB DP Biology hub, the printable checklist, exam preparation and the free 10-minute diagnostics.

Key terms

Term	Meaning
Pathogen	Organism that causes disease: virus, bacterium, fungus or protist (not archaea)
Innate immunity	Fixed, broad response; phagocytes
Adaptive immunity	Specific response that builds memory; lymphocytes
Antigen	Recognition molecule, usually protein or glycoprotein on a pathogen surface
Antibody	Protein made by plasma cells that binds one antigen
Plasma cell	Activated B-cell clone member that secretes antibody
Memory cell	Long-lived lymphocyte that gives immunity
Immunity	Being able to clear an infectious disease from the body, thanks to memory cells
Zoonosis	Infectious disease that transfers from other species to humans
Herd immunity	Transmission impeded because enough of the population is immune

First lines of defence (C3.2.1 to C3.2.3)

• Pathogens: viruses, bacteria, fungi, protists. Examples: cholera (bacterium), COVID-19 (virus), athlete’s foot (fungus), malaria (protist).
• Nature of science: Semmelweis (childbed fever, Vienna) and Snow (cholera, London) used careful observation to control disease.
• Skin: physical barrier (tough, keratinised dead cells) and chemical barrier (acidic, antimicrobial secretions).
• Mucous membranes: sticky mucus traps microorganisms; cilia move it along; enzymes in secretions attack bacteria.
• Clotting steps in order:

1. Platelets release clotting factors.
2. A cascade of reactions follows.
3. Thrombin forms.
4. Thrombin converts soluble fibrinogen to insoluble fibrin.
5. A fibrin mesh traps erythrocytes, making the clot.

Innate versus adaptive (C3.2.4, C3.2.5)

	Innate	Adaptive
Specificity	Broad categories	One pathogen
Changes over life	No	Yes
Memory	No	Yes
Cells	Phagocytes	B-cells and T-cells

Method in steps: phagocytosis

1. Amoeboid movement from blood to infected tissue.
2. Recognise pathogen as foreign.
3. Engulf by endocytosis into a vesicle.
4. Lysosomes fuse and enzymes digest the pathogen.

Adaptive response (C3.2.6 to C3.2.10)

• Lymphocytes circulate in blood and are held in lymph and lymph nodes.
• Huge number of different B-cells, each making one antibody type.
• Antigens on erythrocytes can cause antibody production after transfusion of a different blood group.

Method in steps: clonal selection

1. B-cell binds its specific antigen.
2. A helper T-cell activated by the same antigen contacts it.
3. Activated B-cell divides by mitosis (few B-cells match, so numbers must rise).
4. Clone becomes plasma cells (antibody) and memory cells.
5. Memory cells give faster, larger response on re-exposure.

Worked reminder: 1 cell, division every 6 hours, 96 hours = 16 divisions = 2¹⁶ = 65 536 cells.

HIV and AIDS (C3.2.11, C3.2.12)

• Transmitted in body fluids: blood, semen, vaginal fluids, breast milk.
• Routes: unprotected sex, shared needles, mother to child, unscreened transfusion.
• Infects and kills helper T-cells only (the lymphocytes affected).
• Fewer helper T-cells means fewer activated B-cells, less antibody and opportunistic infections.
• AIDS is the later condition caused by the loss of immune function.

Antibiotics and resistance (C3.2.13, C3.2.14)

Antibiotic target	Why it spares human cells
Cell wall synthesis	Animal cells have no cell wall
Bacterial ribosomes	Differ from eukaryotic ribosomes

• Viruses: no metabolism or ribosomes of their own, live inside host cells, so no target.
• Resistance: random mutation, antibiotic as selection pressure, survivors reproduce, allele frequency rises.
• Slowing it: use only when needed, finish the course, limit routine use in animals.
• Worked reminder: 10 resistant cells, 30-minute generations, 6 hours = 12 doublings = 10 × 2¹² = 40 960.

Zoonoses (C3.2.15)

Disease	Pathogen	Route
Tuberculosis	Mycobacterium bovis (some cases)	Milk or contact with infected cattle
Rabies	Virus	Bite, saliva
Japanese encephalitis	Virus	Mosquito bite
COVID-19	SARS-CoV-2	Animal origin, then spread between humans

Vaccines and herd immunity (C3.2.16, C3.2.17)

• Vaccine = antigens, or DNA or RNA coding for antigens.
• Stimulates memory cells without causing disease.
• Herd immunity: with enough immune people, transmission is greatly impeded and the unvaccinated gain protection.
• Nature of science: media report before evaluation ends; risk is minimal, not zero.

Percentage calculations (C3.2.18)

Calculation	Formula
Percentage change	(new − original) ÷ original × 100
Percentage difference	difference ÷ mean of the two × 100

Worked reminder: admissions 84 and 12 per 100 000. Difference 72, mean 48, so percentage difference 150%. Percentage change from 84 to 12 is −85.7%.

Primary and secondary responses

Graphs of antibody concentration against time are common. Read them in this order.

Feature	First exposure	Second exposure
Lag before antibody appears	Longer	Shorter
Peak concentration	Lower	Higher
Time antibody stays high	Shorter	Longer
Reason	Few matching B-cells, clone must be built	Memory cells already present

Worked reminder: peaks of 40 and 640 arbitrary units. Percentage change = (640 − 40) ÷ 40 × 100 = 1500%, or 16 times higher.

Link to the vaccine idea: a vaccine gives the first exposure safely, and a later booster or real infection gives the second.

Phagocytes and lymphocytes side by side

Cell	System	What it does
Phagocyte	Innate	Moves to infection, engulfs and digests pathogens
B-cell	Adaptive	Binds specific antigen, divides, makes plasma and memory cells
Plasma cell	Adaptive	Secretes antibody
Helper T-cell	Adaptive	Activated by antigen, then activates B-cells
Memory cell	Adaptive	Long-lived, gives faster response next time

Blood group link: erythrocyte surface antigens differ between groups. Transfused blood with the wrong antigens can stimulate antibody production, which is why donor and recipient are matched.

Evaluating data (C3.2.18 and the nature of science points)

Use this checklist when a question asks you to evaluate COVID-19 data or a claim about a vaccine.

1. Is the measure a raw count or a rate per head of population? Rates allow fair comparison between places of different size.
2. How large is the sample? Small samples give less reliable conclusions.
3. Were the compared groups similar apart from the factor tested?
4. Was data collected in the same way over the whole period, for example the same testing policy?
5. Does a correlation show a cause? Several factors often change together.
6. Has the research been published and evaluated, or is it a media report of early findings?
7. Is the risk stated honestly? Side effects of vaccines are minimal but not nil.

Worked reminder: if a vaccinated group shows 15 admissions per 100 000 and an unvaccinated group 60, the percentage difference is 45 ÷ 37.5 × 100 = 120%, while the percentage change from unvaccinated to vaccinated is −75%. State which one you calculated.

Writing good answers

• “Explain” needs a reason as well as a statement. For antibiotics: block a bacterial process, and that process is missing from human cells.
• “Distinguish” needs both sides stated, preferably in the same sentence.
• “Outline” is brief, so give the main steps in order. Clotting is a good example.
• “Evaluate” needs strengths and limitations, then a judgement.
• In calculations, show the formula and the substitution, give the unit or percent sign, and keep sensible significant figures.

Common pairs that get swapped: lysosome and ribosome (digestion versus protein synthesis), endocytosis and exocytosis (phagocytes take pathogens in; plasma cells secrete antibody out), mitosis and meiosis (clonal expansion uses mitosis).

Must-know distinctions

• HIV (virus) is not AIDS (condition).
• Antigen (on pathogen) is not antibody (made by plasma cell).
• Plasma cell secretes antibody; memory cell waits.
• Antibiotics act on bacteria; vaccines prepare the immune system.
• Percentage change needs a starting value; percentage difference does not.

Quick self-test

1. Name the four groups usually called pathogens.
2. Which protein is formed from fibrinogen in a clot?
3. State two differences between innate and adaptive immunity.
4. Which process do phagocytes use to take in a pathogen?
5. Why must an activated B-cell divide many times?
6. Which cells does HIV infect?
7. Why does penicillin not harm human cells?
8. Why are antibiotics useless against influenza?
9. Name one zoonosis spread by mosquitoes.
10. Cases fall from 800 to 200. Calculate the percentage change.
11. Values of 30 and 50 are compared. Calculate the percentage difference.

Answers

1. Viruses, bacteria, fungi and protists.
2. Fibrin (formed by thrombin).
3. Innate: broad, unchanging, no memory. Adaptive: specific, improves, memory.
4. Endocytosis.
5. Few B-cells match the antigen, so cloning is needed for enough plasma cells.
6. Helper T-cells.
7. Human cells have no cell wall.
8. Viruses lack bacterial targets and use host machinery.
9. Japanese encephalitis.
10. (200 − 800) ÷ 800 × 100 = −75%.
11. 20 ÷ 40 × 100 = 50%.

Where marks are usually lost

• Writing “antibodies” in a description of a vaccine.
• Leaving out helper T-cells in B-cell activation.
• Describing phagocytes as specific.
• Saying that bacteria “build immunity” to antibiotics.
• Giving casual contact as a route of HIV transmission.
• Calculating percentage change with the wrong starting value.
• Quoting raw counts when a rate per 100 000 is needed for fair comparison.
• Describing herd immunity without saying who is protected.

Official syllabus

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