AQA GCSE Biology (8461) · 4.3 Infection and response
Mini-Lesson
Infection and response
This mini-lesson walks you through the whole of AQA Topic 4.3 — Infection and response: the four kinds of pathogen and the diseases they cause, how the body defends itself, how vaccination and antibiotics work, how new drugs are developed, plus monoclonal antibodies and plant disease.
Work through each screen, answer the questions as you go and collect ⭐ stars. Items marked HT only are Higher tier; Biology only are separate-science only. Press Start when you're ready.
4.3.1.1 Communicable disease
Four kinds of pathogen
A pathogen is a microorganism that causes communicable (infectious) disease. Pathogens can infect plants or animals and spread by direct contact, by water or by air. There are four types:
The four pathogen types. Bacteria & viruses reproduce rapidly in the body; bacteria make toxins, viruses live and reproduce inside cells, damaging them.
Reducing spread: hygiene, destroying vectors, isolating infected individuals, and vaccination all cut down how easily a pathogen passes on.
Quick check
Spot the pathogen type
?A pathogen is not a living cell, is extremely small, and can only reproduce inside the host's own cells, damaging them. Which type is it?
4.3.1.2–4.3.1.5 Named diseases
The diseases you must name
AQA names one or more diseases for each pathogen type. Learn the pathogen, the host, the symptoms and how it spreads:
Viral — Measles: fever and a red skin rash; can be fatal. Spread by droplets from coughs & sneezes. Most young children are vaccinated.
Viral — HIV: a flu-like illness; if not controlled by antiretroviral drugs it attacks immune cells, leading to AIDS. Spread by sexual contact or exchange of body fluids (e.g. sharing needles).
Viral — Tobacco mosaic virus (TMV): a plant pathogen giving a "mosaic" discolouration on leaves; reduces photosynthesis and growth.
Bacterial — Salmonella: food poisoning (fever, cramps, vomiting, diarrhoea) from toxins. Spread in food; UK poultry are vaccinated.
Bacterial — Gonorrhoea: an STD (yellow/green discharge, pain on urinating). Spread by sexual contact; treat with antibiotics or use a condom.
Fungal — Rose black spot: black/purple spots on rose leaves which yellow and drop, cutting photosynthesis. Spread by water/wind; treat with fungicides or remove leaves.
Protist — Malaria: recurrent fevers, can be fatal. The protist's life cycle uses the mosquito vector; control by stopping mosquitoes breeding and using nets.
Spotting the host: TMV and rose black spot infect plants; the others infect animals/humans.
Sort it
Match the disease to its pathogen
Tap a disease, then tap the pathogen-type box it belongs in.
🦠 Bacteria
🧬 Virus
🍄 Fungus
🦟 Protist
4.3.1.6 Human defences
The body's first line: non-specific defences
Before the immune system gets involved, the body uses non-specific barriers that work against all pathogens:
The four non-specific defences AQA names: skin, nose, trachea & bronchi (mucus + cilia) and the stomach (acid).
Why "non-specific"? These barriers don't target one particular pathogen — they block or destroy anything that arrives.
4.3.1.6 The immune system
White blood cells fight back
If a pathogen gets past the barriers, the immune system tries to destroy it. White blood cells defend the body in three ways:
White blood cells: phagocytosis (engulf & digest), antibody production (lock onto a specific antigen), and antitoxin production (neutralise toxins).
Watch out: an antibody is specific — it fits one antigen like a key in a lock. An antitoxin counteracts the toxin a bacterium releases — it does not engulf the pathogen.
Sort it
Name the defence
Tap how the body is defending itself in each case.
4.3.1.7 Vaccination
How vaccination works
A vaccine introduces small quantities of dead or inactive forms of a pathogen. This stimulates white blood cells to make the right antibodies — without you getting ill:
If the real pathogen later invades, white blood cells respond quickly with the correct antibodies, preventing infection.
Herd immunity: if a large proportion of the population is immunised, the pathogen can't spread easily — protecting even the few who aren't vaccinated. Watch out: the vaccine uses dead/inactive pathogen, so it can't give you the disease.
Quick check
The big idea of vaccination
?Why does a vaccine protect you against a disease without making you ill?
4.3.1.8 Antibiotics & painkillers
Antibiotics, painkillers & resistance
Antibiotics (e.g. penicillin) cure bacterial disease by killing the infective bacteria inside the body. A specific bacterium needs the specific antibiotic.
Antibiotics cannot kill viruses — viruses live inside your cells, so it is hard to kill them without harming your own tissues.
Painkillers and other medicines treat the symptoms only — they do not kill the pathogen.
Antibiotic resistance: overuse has let resistant strains of bacteria emerge, which is a major concern.
Key misconception: antibiotics do not work on viral infections like colds or flu.Quick check
Why antibiotics fail on viruses
?A doctor refuses to prescribe antibiotics for a patient's cold. Which is the best reason?
4.3.1.9 Discovery & development of drugs
Where drugs come from — and how they're tested
Traditionally, drugs were extracted from plants and microorganisms:
Digitalis (a heart drug) — from foxgloves.
Aspirin (a painkiller) — from willow.
Penicillin — discovered by Alexander Fleming from the Penicillium mould.
New drugs are tested for toxicity (is it harmful?), efficacy (does it work?) and dose (how much?):
Double-blind trial: some patients get a placebo, and neither the patient nor the doctor knows who got the real drug — this removes bias.Quick check
Reading a drug trial
?In a trial, some patients are given a placebo and neither the patients nor the doctors know who received the real drug. What is this called, and why is it done?
HT onlyBiology only
Monoclonal antibodies
Monoclonal antibodies are made from a single clone of cells, so they are all identical and bind to one specific antigen — letting them target a particular chemical or cell. They are made using hybridoma cells:
Lymphocyte + tumour cell → hybridoma, which both divides and makes the antibody. Clone it to make large amounts of identical antibody.
Uses: pregnancy tests; measuring hormone/chemical levels or detecting pathogens; locating molecules with a fluorescent dye in research; and treating cancer (the antibody carries a radioactive substance, toxic drug or growth-blocker to the cancer cells only).
Watch out: monoclonal antibodies cause more side effects than expected, so they aren't yet as widely used as first hoped.
Quick check
Making a monoclonal antibody
?A mouse lymphocyte is fused with a tumour cell. Why is the resulting hybridoma so useful for making monoclonal antibodies?
Biology only
Plant disease & defences
Plants are infected by viral, bacterial and fungal pathogens, and by insects (aphids). You only need: TMV (viral), rose black spot (fungal) and aphids (insects).
Detecting diseaseHT: stunted growth, spots on leaves, rot, growths, malformed stems/leaves, discolouration, or visible pests.
Identifying itHT: use a gardening manual/website, send the plant to a lab, or use a testing kit with monoclonal antibodies.
Ion deficiencies:nitrate deficiency → stunted growth (nitrate is needed for protein synthesis); magnesium deficiency → chlorosis (magnesium is needed to make chlorophyll).
Plants defend themselves physically, chemically and mechanically:
Three categories of plant defence — physical barriers, chemical deterrents, and mechanical adaptations.Quick check
Diagnosing a plant
?A gardener notices the older leaves of a plant have turned yellow between the veins (chlorosis), but there are no pests or spots. Which deficiency is the most likely cause?