Diseases and Immunity
Pathogens and transmissible diseases; the five body defences; disease control measures; and Extended: antigens, antibodies, active vs passive immunity, the three-step vaccination process, memory cells, breast-feeding and passive immunity, and the cholera toxin mechanism.
Diseases and Immunity
CORE EXTENDEDPathogens and Transmissible Diseases
| Term | Definition | Examples |
|---|---|---|
| Pathogen | A disease-causing organism | Bacteria (cholera, tuberculosis), viruses (influenza, HIV), fungi, protoctists (Plasmodium — malaria) |
| Transmissible disease | A disease in which the pathogen can be passed from one host to another | Cholera, influenza, COVID-19, malaria, HIV/AIDS |
Routes of Transmission
| Type | Route | Examples |
|---|---|---|
| Direct contact | Physical contact between infected and uninfected person; through blood and other body fluids | HIV (blood/body fluids); sexually transmitted infections; skin infections |
| Indirect contact | Via contaminated surfaces, food, animals, or air | Contaminated water (cholera); contaminated food (Salmonella); animals/vectors (mosquitoes → malaria); airborne droplets (influenza, COVID-19) |
Body Defences Against Pathogens
| Defence | How it works |
|---|---|
| Skin | Physical barrier — intact skin prevents most pathogens from entering the body. If broken (cuts, burns), infection risk increases dramatically. |
| Hairs in the nose | Filter out dust particles and some pathogens from inhaled air before they reach the lungs |
| Mucus | Sticky secretion in the respiratory tract traps dust, pollen, and pathogens; cilia beat to move mucus (with trapped particles) up and out of the airways |
| Stomach acid (HCl) | Very low pH (~2) kills most bacteria and pathogens that are swallowed with food or drink |
| White blood cells | Phagocytes engulf pathogens that enter the body; lymphocytes produce antibodies specific to the pathogen's antigens |
Controlling the Spread of Disease
| Measure | How it reduces disease spread |
|---|---|
| Clean water supply | Prevents waterborne diseases (e.g. cholera, typhoid) by removing or killing pathogens before water is consumed |
| Hygienic food preparation | Thorough cooking kills pathogens; refrigeration slows microbial growth; preventing cross-contamination stops pathogens entering food |
| Good personal hygiene | Handwashing removes pathogens from hands before they enter the body or contaminate food/surfaces; reduces faecal-oral transmission |
| Waste disposal | Safe removal of rubbish reduces breeding sites for disease vectors (flies, rats, mosquitoes) and prevents water/food contamination |
| Sewage treatment | Removes or destroys pathogens from human waste before it enters water sources, preventing waterborne disease outbreaks (details of stages not required) |
Which body defence specifically kills pathogens that have been swallowed with food?
- A. Hairs in the nose
- B. Skin
- C. Stomach acid
- D. Mucus in the airways
Antigens and Antibodies — Extended
| Term | What it is | Key property |
|---|---|---|
| Antigen | A molecule (usually a protein) on the surface of a pathogen | Each pathogen has its own antigens with specific shapes — these are what the immune system recognises as "foreign" |
| Antibody | A protein produced by lymphocytes (B cells) in response to a specific antigen | Specific antibodies have complementary shapes that fit specific antigens — like a lock and key. They bind to antigens, leading to direct destruction of pathogens or marking them for phagocytosis. |
Active vs Passive Immunity — Extended
| Feature | Active immunity | Passive immunity |
|---|---|---|
| Definition | Defence against a pathogen by antibody production in the body | Short-term defence by antibodies acquired from another individual |
| How gained | After infection by a pathogen; or by vaccination | Across the placenta (from mother to foetus); in breast milk; or by injection of antibodies |
| Memory cells produced? | ✓ Yes — gives long-term immunity | ✗ No — therefore only short-term protection |
| Duration of protection | Long-term (often lifelong for natural infection) | Short-term (weeks to months — until antibodies break down) |
| Speed of protection | Slower — immune system must respond and build up antibodies | Immediate — ready-made antibodies provided |
Vaccination — Three-Step Process — Extended
Step 1: Weakened (attenuated) pathogens, or their antigens alone, are introduced into the body (by injection or orally).
Step 2: The antigens stimulate an immune response — lymphocytes recognise the foreign antigens and produce specific antibodies.
Step 3: Memory cells are produced. These remain in the blood long after the infection has been cleared. If the same pathogen is encountered again in the future, memory cells enable a much faster and stronger secondary immune response, preventing disease.
When a high proportion of a population is vaccinated, the pathogen cannot spread easily — most potential hosts are immune. This is called herd immunity. It protects even those who cannot be vaccinated (e.g. newborns, immunocompromised individuals) because the pathogen is unlikely to reach them.
Passive Immunity and Breast-feeding — Extended
During pregnancy, antibodies from the mother’s blood cross the placenta into the foetal circulation. The newborn therefore has a temporary set of maternal antibodies that protect against common infections in the first weeks of life.
Breast milk (especially the first milk — colostrum) contains maternal antibodies. When the infant feeds, these antibodies are absorbed and provide passive immunity against pathogens the mother has encountered. This is why breast-feeding is important for infant immunity — especially in the first months before the infant’s own immune system is fully developed.
Cholera — Case Study — Extended
Cause: Cholera is caused by a bacterium (Vibrio cholerae), transmitted in contaminated water.
Mechanism:
1. The cholera bacterium colonises the small intestine and produces a toxin.
2. The toxin causes massive secretion of chloride ions (Cl⁻) into the lumen of the small intestine.
3. The high Cl⁻ concentration creates a lower water potential in the gut lumen.
4. Water moves by osmosis from the blood and intestinal cells into the gut lumen (down the water potential gradient).
5. Result: severe diarrhoea, rapid dehydration, and loss of ions from the blood.
6. If untreated, dehydration and ion imbalance can be fatal within hours. Treatment = oral rehydration salts (water + ions + glucose).
A child is vaccinated against measles at 12 months old. Explain how the vaccine produces long-term immunity. Use the terms: antigen, lymphocyte, antibody, memory cell. [4 marks]
- The vaccine contains weakened measles pathogens or their antigens [1 mark]
- The antigens stimulate lymphocytes to produce specific antibodies that bind to those antigens [1 mark]
- Memory cells are produced during this immune response [1 mark]
- If the child later encounters the measles virus, memory cells enable a rapid and stronger secondary immune response — antibodies are produced quickly before the disease can develop, giving long-term immunity [1 mark]
Explain how the cholera bacterium causes dehydration in an infected person. [4 marks]
- The cholera bacterium produces a toxin in the small intestine [1 mark]
- The toxin causes secretion of chloride ions (Cl⁻) into the gut lumen [1 mark]
- This lowers the water potential of the gut contents / creates a higher solute concentration in the gut [1 mark]
- Water moves by osmosis from blood/body cells into the gut lumen → severe diarrhoea → rapid dehydration and loss of ions from the blood [1 mark]
Comprehensive Practice Questions
Mixed questions across Topic 10.
Cholera spreads rapidly in areas affected by flooding. Which route of transmission best explains this?
- A. Direct contact between infected individuals
- B. Indirect transmission via contaminated water supplies
- C. Airborne droplets exhaled by infected people
- D. Animal vectors such as mosquitoes
(a) Describe how mucus in the respiratory tract defends against pathogens. [2 marks]
(b) Explain why sewage treatment is important in controlling the spread of disease. [2 marks]
(c) State one other measure that helps control disease spread and explain how it works. [2 marks]
- (a) Mucus is sticky and traps dust, pollen, and pathogens [1 mark]; cilia (on ciliated cells) beat to move mucus with trapped particles up and out of the airways, preventing them reaching the lungs [1 mark]
- (b) Sewage contains pathogens from human waste [1 mark]; if not treated, sewage contaminates water supplies and food, causing waterborne diseases such as cholera and typhoid; treatment removes or destroys these pathogens before they reach drinking water [1 mark]
- (c) Any one of the five measures with a valid explanation; e.g. clean water supply: removes pathogens from drinking water, preventing waterborne diseases [2 marks]; personal hygiene (handwashing): removes pathogens from hands before food contact or touching the mouth, preventing faecal-oral transmission [2 marks]
A newborn baby has antibodies against several diseases even before receiving any vaccinations. These antibodies were received via the placenta from the mother. Which statement about this type of immunity is correct?
- A. It is active immunity because the antibodies came from a living organism
- B. It gives long-term protection because memory cells were transferred with the antibodies
- C. It is passive immunity and will only last a short time as no memory cells are produced
- D. It is active immunity because the baby’s own lymphocytes made the antibodies
Compare active and passive immunity, with reference to: how each is obtained, whether memory cells are involved, and the duration of protection. [5 marks]
- Active immunity: gained after infection by the pathogen or by vaccination [1 mark]; passive immunity: antibodies are acquired from another individual — via the placenta, breast milk, or injection [1 mark]
- In active immunity, the individual’s own lymphocytes respond to antigens and produce antibodies AND memory cells [1 mark]; in passive immunity, ready-made antibodies are received and no memory cells are produced [1 mark]
- Active immunity provides long-term protection (often lifelong); passive immunity provides only short-term protection (weeks to months, until the acquired antibodies break down) [1 mark]
High-Frequency Mistakes — Topic 10 Overall
- 🔄Confusing antibodies and antigensAntigens are molecules on the surface of pathogens — they are what the immune system recognises as foreign. Antibodies are proteins made by your own lymphocytes in response to antigens. Mnemonic: AntiGEN = GENerated by the pathogen; AntiBODY = produced BY your body.
- 💉Saying antibodies "kill" pathogens directlyAntibodies bind to antigens leading to direct destruction of the pathogen OR marking it for destruction by phagocytes. Saying simply "kill pathogens" omits the mechanism and may not receive full marks.
- 🔐Ext: Saying vaccines inject antibodiesVaccines inject weakened pathogens or their antigens — this stimulates the body to produce its own antibodies and memory cells. Injecting ready-made antibodies would be passive immunity (antibody therapy), not vaccination.
- 📌Ext: Saying passive immunity is long-termPassive immunity is always short-term — the acquired antibodies break down over weeks to months. Because no memory cells are produced, there is no long-term protection. Only active immunity (infection or vaccination) produces memory cells and long-term protection.
- 👶Ext: Forgetting memory cells in vaccination answersFor full marks on "explain how a vaccine produces long-term immunity", all four steps are needed: antigen introduced → lymphocytes produce antibodies → memory cells formed → faster secondary response if pathogen encountered again. Missing memory cells typically loses 1–2 marks.
- 🪺Ext: Stopping the cholera chain at "diarrhoea"The full required chain is: toxin → Cl⁻ secretion → lower water potential in gut → water moves by osmosis into gut → diarrhoea → dehydration + ion loss. Papers award marks at each step; stopping at just "diarrhoea" without explaining the osmosis mechanism will lose the majority of marks.
- 💧Saying skin "absorbs" pathogensIntact skin is a barrier that prevents pathogens from entering — it does not absorb them. Only when skin is broken (cuts, wounds) can pathogens enter. This distinction is important for describing skin as a body defence.
Highest-yield Core items: the five body defences with explanations (not just a list — explain how each prevents infection); the five disease control measures with mechanisms. For Extended: the active vs passive immunity comparison table (especially memory cells = active only; short-term = passive); the three-step vaccination mechanism using the four key terms (antigen, lymphocyte, antibody, memory cell); and the cholera toxin chain (Cl⁻ → osmosis → water into gut → diarrhoea/dehydration). The cholera question appears frequently in Paper 4 because it combines Topic 10 knowledge with Topic 3 (osmosis) — a cross-topic application the syllabus specifically tests.