Key Idea: Topic 3.7 is the story of how the body stops pathogens — bacteria, viruses, fungi and protists — from making us ill. It is best remembered as a set of layers, each one taking over if the layer before it is breached. Layer 1 — keep them out. The skin, mucous membranes and stomach acid are primary defences that block any pathogen from entering. Blood clotting seals cuts so the skin barrier is not left open. Layer 2 — the fast, general attack (innate). If a pathogen gets in, phagocytes engulf and digest any invader at once — fast, non-specific, with no memory. Layer 3 — the slow, targeted attack (adaptive). Lymphocytes then mount a specific response: helper T-cells activate B-cells, B-cells make antibodies against one pathogen, and memory cells are kept so the next attack is much faster. That memory is exactly what vaccines exploit — and exactly what HIV destroys. Finally, antibiotics help us kill bacteria (but not viruses), though over-use breeds resistance, and some diseases jump to us from animals (zoonoses). This topic is a regular on Paper 1 (identify an innate vs adaptive cell, read an antibody or helper-T-cell graph, spot a zoonosis) and a favourite on Paper 2 / Paper 3 (explain phagocytosis, the antibody response, how a vaccine works, or how resistance evolves).
🦠 Pathogens & the primary defences
A pathogen is an organism (or particle) that causes disease — a bacterium, virus, fungus or protist. Pathogens harm us in two ways: by damaging the cells they infect, or by releasing toxins (poisons) that disrupt how cells work. The body's first job is to stop them getting in at all. The primary (first-line) defences are non-specific barriers — they work against any pathogen — and there are three to know.
| Primary defence | Physical or chemical? | How it blocks pathogens |
|---|---|---|
| Skin | Physical | a tough, dry wall of dead cells that pathogens cannot cross while it is unbroken |
| Mucous membranes | Physical (mucus + cilia) | sticky mucus traps pathogens; in the airways cilia sweep it away |
| Stomach acid | Chemical | a very low pH kills most pathogens swallowed in food or mucus |
Skin and mucus = physical (they block). Stomach acid = chemical (it kills). And the exam pattern is always the same: weaken a barrier → more infection. A cut breaks the skin; smoking damages the cilia; acid-reducing medicine raises the stomach pH — in every case, more pathogens get past.
🩹 Blood clotting & sealing wounds
When the skin is cut, clotting seals the wound — which does two jobs at once: it stops blood loss and it bars pathogens from the tissues below. So clotting is part of the first line of defence. Clotting happens as a cascade — a chain where each step switches on the next — and it only starts where a vessel is damaged (no damage → no clot).
| Step | What happens | Why it matters |
|---|---|---|
| 1 — vessel cut | a damaged vessel exposes its surface to the blood | the trigger — clotting only starts at a wound |
| 2 — platelets act | platelets stick, clump and release clotting factors | form a temporary plug and start the cascade |
| 3 — thrombin made | clotting factors turn prothrombin into thrombin | thrombin is the enzyme that builds the mesh |
| 4 — fibrin mesh | thrombin turns soluble fibrinogen into insoluble fibrin | a tangled mesh forms across the wound |
| 5 — clot & scab | the mesh traps blood cells and dries into a scab | seals the cut, stops bleeding, blocks pathogens |
Walk the chain: cut → platelets → clotting factors → thrombin → fibrin mesh → clot/scab. The one-line chemistry: thrombin turns SOLUBLE fibrinogen into INSOLUBLE fibrin. And for an infection mark you must reach the barrier point — the clot keeps pathogens out, not just 'stops bleeding'.
⚡ Innate immunity — phagocytes
If a pathogen gets past the barriers, the first responders are the cells of the innate immune system. The key innate cells are phagocytes (such as macrophages and neutrophils) — white blood cells that engulf and digest invaders. The innate response is fast, non-specific (it attacks any pathogen) and has no memory — it works the same way every time. The process is phagocytosis: engulf the pathogen → enclose it in a vacuole → digest it with enzymes.
| Step of phagocytosis | What the phagocyte does | Why it matters |
|---|---|---|
| 1. Recognise | detects the pathogen as foreign (non-self) | done non-specifically — to any pathogen, not just one type |
| 2. Engulf | surrounds the pathogen and takes it inside | the cell membrane pulls the pathogen in |
| 3. Enclose | traps the pathogen in a membrane-bound vacuole | seals it away inside the cell |
| 4. Digest | enzymes break the pathogen down | the pathogen is destroyed and removed |
Phago = eat. A phagocyte is an 'eating cell': it engulfs the pathogen, bags it in a vacuole, and digests it. Innate = fast, non-specific, no memory (phagocytes). In a cell-count table, the phagocyte count rises FIRST — because the innate response is the fast one.
🎯 Adaptive immunity — antibodies & memory
The second, more powerful defence is adaptive (specific) immunity, carried out by lymphocytes. It is specific because it targets one pathogen, recognised by a molecule on its surface called an antigen. Antibody production is triggered when a lymphocyte detects the antigen. Then helper T-cells activate B-cells; the activated B-cells make antibodies — Y-shaped proteins whose binding sites fit one antigen. Crucially, some B-cells become memory cells, so the same pathogen is dealt with far faster next time.
The antibody is the signature weapon of the specific (adaptive) defence: a Y-shaped protein whose two arm-tips (the variable region) are shaped to fit ONE antigen, like a key in one lock. B-cells make it, helper T-cells switch on the B-cells that make it, memory cells let it be made fast the second time — and that is exactly the response a vaccine teaches and that HIV destroys.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Cell / molecule | Its job | Key point |
|---|---|---|
| Helper T-cell | detects the antigen and ACTIVATES other cells | its main job is to switch on B-cells — it does NOT make antibodies |
| B-cell | once activated, divides and makes antibodies | the cell that actually produces the antibodies |
| Antibody | binds one specific antigen, tagging the pathogen | Y-shaped; its variable tips fit one antigen → specific |
| Memory cell | a long-lived lymphocyte kept after the infection | gives a faster, larger response if the same pathogen returns |
T for Trigger (helper T-cells activate the response). B for Builder (B-cells build antibodies). And memory cells make the second response the fast one — a small/slow primary curve the first time, a tall/early secondary curve the next.
🛡️ HIV/AIDS & vaccination
Two opposite stories, both turning on the helper T-cell. HIV (Human Immunodeficiency Virus) infects and destroys helper T-cells. Because helper T-cells activate the antibody-making B-cells, their loss weakens the whole immune system; over years the count falls so low that the immune system collapses — this late stage is AIDS, and the person suffers opportunistic infections. A vaccine does the reverse — it strengthens immunity in advance. It gives a harmless antigen (a weakened, dead or partial pathogen) that triggers a primary response and makes memory cells, but does not cause the disease. If the real pathogen later invades, those memory cells drive a fast, large secondary response — immunity without ever being ill.
Vaccine — strengthens immunity: Gives a **harmless antigen** (no disease). Triggers a **primary response** + **memory cells**. Later: a **fast, large secondary response**. Result: **immunity** — pathogen cleared before symptoms.
HIV — destroys immunity: Infects and **destroys helper T-cells**. **Antibody production falls** over years. Immune system **collapses** → AIDS. Result: **opportunistic infections** and cancers.
Keep HIV and AIDS distinct: HIV is the virus; AIDS is the late stage when the immune system has collapsed. Vaccine = a safe rehearsal — the body practises on a harmless antigen and keeps a memory. HIV = sabotage — it removes the very helper T-cell that runs the whole response (which is why there is still no simple vaccine for it).
💊 Antibiotics, resistance & zoonoses
An antibiotic is a medicine that kills bacteria by attacking something only bacteria have — for example a cell wall to build, or their own ribosomes and enzymes. Human cells lack these exact targets, so the drug harms the bacteria and not us. A virus is not a cell: it has no cell wall, no ribosomes and no metabolism of its own. With no bacterial target to attack, an antibiotic does nothing to a virus — so flu and colds cannot be treated with antibiotics. Resistance evolves by natural selection: a few bacteria are already resistant (often from a mutation), the antibiotic kills the rest, and the resistant survivors reproduce until the strain is common — so the drug stops working. Finally, a zoonosis is a disease that passes directly from an animal to a human (rabies, some TB, Japanese encephalitis).
| Question | Short answer | Key reason |
|---|---|---|
| Treats bacteria? | Yes | there is a bacteria-only target (cell wall, ribosomes, enzymes) |
| Treats viruses? | No | a virus is not a cell — no target to attack |
| How does resistance arise? | Natural selection | resistant survivors reproduce; the strain becomes common |
| What is a zoonosis? | Animal → human disease | it passes directly from an animal to a person |
Antibiotic = anti-bacteria (no effect on viruses — name the missing target). Resistance is selected, not learned — never say a bacterium 'tries' or 'learns' to resist. Zoonosis = a disease from an animal ('zoo' = animal).
✍️ Worked examples
IB-style question — a weakened primary defence
Some patients take medicine that reduces the amount of acid their stomach produces. Suggest why these patients are more likely to develop bacterial infections of the gut. [2]
How to score both marks:
Identify the weakened barrier. Stomach acid is a chemical primary defence: its low pH normally kills swallowed pathogens. The medicine raises the pH, so the stomach is less acidic and kills fewer.
Link to more infection. With less acid, more swallowed bacteria survive and reach the intestines, where they multiply and cause an infection. (Mark 1: less acid / higher pH kills fewer pathogens. Mark 2: more bacteria survive and reach the gut, causing infection.)
Stomach acid normally kills swallowed pathogens; less acid (higher pH) kills fewer, so more bacteria survive, reach the gut and cause infection.
IB-style question — how a clot prevents infection
A gardener gets a deep cut on her hand. Explain how blood clotting at the wound helps to prevent infection. [3]
How to score all three marks:
Platelets start a clot. Platelets stick to the damaged vessel, clump together and release clotting factors, which trigger the cascade that forms a clot.
The clot seals the cut. Clotting factors lead to thrombin, which turns fibrinogen into a fibrin mesh that traps blood cells, sealing the wound and drying into a scab.
The seal blocks pathogens. The clot / scab is a physical barrier, so bacteria and other pathogens cannot enter the tissues through the cut. (Award 1 mark per distinct point: platelets / clotting factors → fibrin clot seals the wound → barrier keeps pathogens out.)
Platelets release clotting factors that form a fibrin clot, sealing the cut; the clot / scab is a physical barrier, so pathogens cannot enter through the broken skin.
IB-style question — destroying a pathogen by phagocytosis
A pathogen enters the body and is destroyed by a phagocyte of the innate immune system. Outline how the phagocyte destroys the pathogen by phagocytosis. [4]
How to score all four marks:
Recognise. The phagocyte recognises the pathogen as foreign (non-self) — and does this non-specifically, so it works on any pathogen.
Engulf. The phagocyte's cell membrane surrounds the pathogen and takes it inside the cell.
Enclose. The pathogen is sealed inside a membrane-bound vacuole.
Digest. Enzymes are released into the vacuole and break the pathogen down, destroying it. (Award 1 mark per ordered step: recognise non-specifically → engulf → enclose in a vacuole → digest with enzymes.)
The phagocyte recognises the pathogen as foreign (non-specifically), engulfs it, encloses it in a vacuole and digests it with enzymes.
IB-style question — what triggers antibodies, and the helper T-cell's role
A person is infected by a bacterium for the first time. State the event that triggers antibody production, and outline the role of helper T-cells in the response. [3]
How to score all three marks:
Name the trigger. Antibody production is triggered when a lymphocyte detects the antigen of the invading pathogen.
Give the helper T-cell's job. The helper T-cell activates other immune cells — in particular it activates the B-cells.
Link to antibodies. The activated B-cells then divide and make antibodies specific to that antigen. (Mark 1: antigen detected. Mark 2: helper T-cell activates B-cells. Mark 3: B-cells produce antibodies. Saying the helper T-cell makes antibodies loses a mark.)
The trigger is a lymphocyte detecting the pathogen's antigen; the helper T-cell then activates the B-cells, which divide and produce specific antibodies.
IB-style question — read a falling helper T-cell graph
A graph shows that, in an untreated patient, the number of helper T-cells in the blood falls steadily over the nine years following HIV infection. Using the graph, explain a likely symptom of this patient nine years after infection. [3]
How to score all three marks:
Read the trend. After nine years the helper T-cell count is very low (it has fallen steadily since infection).
Link low helper T-cells to weak immunity. With few helper T-cells, B-cells are not activated, so few antibodies are made and the immune response is weak.
Name the resulting symptom. The patient is likely to suffer frequent or unusual (opportunistic) infections — illnesses a healthy immune system would normally fight off. (Mark 1: helper T-cells very low. Mark 2: weak immune response / antibody production reduced. Mark 3: opportunistic infection named — the patient has reached AIDS.)
Helper T-cell numbers are very low after nine years, so the immune response is weak and few antibodies are made; the patient is therefore likely to suffer opportunistic infections — they have reached AIDS.
IB-style question — why the same antibiotic later fails
A patient with a bacterial infection is treated with an antibiotic and recovers. Months later they catch a second infection with the same species, but the same antibiotic does not work. Explain, in terms of natural selection, why the antibiotic failed. [4]
How to score all four marks:
Variation. Within the bacterial population a few bacteria already carry a gene (often from a random mutation) that makes them resistant to the antibiotic.
Selection. When the antibiotic is used, it kills the non-resistant bacteria, but the resistant ones survive.
Reproduction. The resistant survivors reproduce and pass the resistance gene to their offspring, so the proportion of resistant bacteria rises.
The strain becomes common. The second infection is now caused by a mostly resistant strain, so the same antibiotic no longer kills these bacteria and fails. (Award 1 mark each: variation → selection → reproduction of survivors → resistant strain common.)
A few bacteria were already resistant (mutation); the antibiotic killed the non-resistant ones; the resistant survivors reproduced; the strain became mostly resistant, so the same antibiotic no longer works — natural selection.
✅ Quick self-check
Tap each card to check yourself across all six micros.
What are the three primary defences, and which is chemical? The skin, the mucous membranes (mucus + cilia) and stomach acid. Stomach acid is the chemical barrier — its low pH kills swallowed pathogens; the skin and mucus are physical barriers. Weaken any barrier (cut skin, damaged cilia, less acid) and infection becomes more likely.
How does a blood clot form, and how does it prevent infection? Cut vessel → platelets stick and release clotting factors → thrombin is made → thrombin turns soluble fibrinogen into insoluble fibrin → the fibrin mesh traps cells, forming a clot and scab. The clot seals the wound, so it stops blood loss AND acts as a barrier keeping pathogens out.
What are the features and process of innate immunity? Innate = fast, non-specific, no memory. Its cells are phagocytes (macrophages, neutrophils). Phagocytosis: recognise the pathogen non-specifically → engulf it → enclose it in a vacuole → digest it with enzymes. The phagocyte count rises first in a data table during infection.
How is the adaptive response triggered, and what do helper T-cells do? A lymphocyte detecting the antigen triggers it. Helper T-cells ACTIVATE B-cells (they do not make antibodies); activated B-cells make Y-shaped antibodies that fit one antigen. Memory cells stay behind, giving a faster, larger secondary response on re-exposure.
How does HIV cause AIDS, and how does a vaccine give immunity? HIV destroys helper T-cells; without them antibody production fails and, when the count falls low enough, the immune system collapses (AIDS) with opportunistic infections. A vaccine gives a harmless antigen → primary response → memory cells, so a real infection meets a fast secondary response — immunity without illness.
Why do antibiotics fail on viruses, and how does resistance evolve? Antibiotics attack a bacteria-only target (cell wall, ribosomes, enzymes); a virus is not a cell and has no such target, so the drug does nothing. Resistance evolves by natural selection: a few bacteria are already resistant, the antibiotic kills the rest, and the resistant survivors reproduce until the strain is common.
Visual recap of the adaptive layer: each antibody binds only its matching antigen, tagging that one pathogen for destruction. Specificity (one antibody → one antigen) plus memory (faster next time) is what makes adaptive immunity so powerful — and is the whole basis of vaccination.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Exam Tips
- Think in layers: barriers (keep out) → innate phagocytes (fast, non-specific) → adaptive lymphocytes (slow, specific, with memory). Most questions ask you to place a defence in the right layer.
- Skin and mucus are PHYSICAL barriers; stomach acid is the CHEMICAL one. The exam pattern is 'weaken a barrier → more infection' (cut skin, damaged cilia, less stomach acid).
- Learn the clotting cascade as a fixed chain: cut → platelets / clotting factors → thrombin → fibrin → clot. The one-line chemistry is thrombin turns SOLUBLE fibrinogen into INSOLUBLE fibrin (don't swap them).
- For 'how does a clot prevent INFECTION?', reach the barrier point — the clot seals the cut so pathogens cannot enter — not just 'stops bleeding'.
- Asked which cell is innate? It is a phagocyte (macrophage / neutrophil) — never a lymphocyte, B-cell or T-cell. Describe phagocytosis as engulf → vacuole → digest, and use the word non-specific.
- In a cell-count graph, the phagocyte (innate) numbers rise FIRST; lymphocyte (adaptive) numbers rise later. Quote a value to score the data mark.
- Antibody production is triggered by a lymphocyte detecting an ANTIGEN. Helper T-cells ACTIVATE B-cells; they do not make antibodies. The B-cell makes the antibody — keep the jobs separate.
- On an antibody graph: a higher, faster second peak = memory cells / secondary response; no antibodies at the start = no prior exposure to that antigen.
- Keep HIV (the virus) and AIDS (the late stage) distinct. For the falling helper-T-cell graph, read off the low count, then link it to a weak immune system and opportunistic infection.
- For 'how a vaccine works', give the chain: harmless antigen → primary response → MEMORY cells → faster, larger secondary response. The memory cell is the marking point students forget.
- To explain why antibiotics fail on a virus, NAME a bacteria-only target the virus lacks (cell wall / ribosomes / metabolism) — don't just say 'viruses are different'.
- Resistance is natural selection: variation → the antibiotic selects → survivors reproduce. Never say a bacterium 'becomes' or 'learns' to be resistant on purpose. A zoonosis = animal-to-human transmission.
Key Idea: Defence against disease works in layers. Primary defences — the skin and mucous membranes (physical) and stomach acid (chemical) — keep pathogens out; blood clotting (platelets → clotting factors → thrombin → fibrin mesh) seals cuts so the skin barrier is not left open, blocking pathogens as well as stopping bleeding. If a pathogen gets in, the innate system attacks first: phagocytes destroy any invader by phagocytosis (engulf → vacuole → digest) — fast, non-specific, no memory. Then the adaptive system mounts a specific response: an antigen is detected, helper T-cells activate B-cells, B-cells make antibodies, and memory cells make the next response far faster. That memory is the basis of vaccination (a harmless antigen builds memory without illness) — and its loss is the tragedy of HIV/AIDS (the virus destroys the helper T-cells the whole response depends on). Finally, antibiotics kill bacteria by hitting a bacteria-only target but do nothing to viruses; over-use selects for resistance by natural selection; and some diseases reach us straight from animals as zoonoses.