Infection and response: GCSE Biology

Infection and response is the third topic of AQA GCSE Biology and Combined Science. It covers pathogens, the microorganisms that cause communicable (infectious) diseases; the named diseases you need to know; how the body defends itself; and how we prevent and treat disease with vaccines, antibiotics and new medicines. Biology students also study monoclonal antibodies and plant diseases. This page summarises every part.

Pathogens and how diseases spread

Pathogens are microorganisms that cause infectious disease. AQA names four types: viruses, bacteria, protists and fungi. They can infect plants or animals and spread by direct contact, by water or by air.

• Bacteria can reproduce rapidly inside the body and may release poisons (toxins) that harm tissues and cause the feeling of illness.
• Viruses live and reproduce inside cells, damaging them.

The diseases below show the main ways of reducing spread: vaccination, hygiene when preparing food, barrier contraception, stopping vectors (organisms that carry a pathogen, such as mosquitoes), and removing infected leaves.

The diseases you need to know

AQA notes that gonorrhoea was easily treated with penicillin until many resistant strains appeared. For more on malaria, see is malaria a protist?

How the body defends itself

Non-specific defences

Sort the body's defences in this question from the Human Physiology course.

The immune system

If a pathogen gets into the body, the immune system tries to destroy it. White blood cells help in three ways: phagocytosis (engulfing and digesting pathogens), antibody production and antitoxin production. See the white blood cell diagram and antibodies guides.

Vaccination

A vaccine contains small quantities of a dead or inactive form of a pathogen. This triggers white blood cells into making antibodies. Should the same pathogen enter the body later, the white blood cells respond quickly with the correct antibodies, preventing infection. Immunising a large proportion of a population also reduces the spread of the pathogen, which protects people who are not vaccinated.

Antibiotics and painkillers

Specific bacteria should be treated with specific antibiotics. Drugs that kill viruses are hard to develop, because they must not damage the body's own tissues, which the viruses live inside.

Discovering and developing drugs

Traditionally, drugs came from plants and microorganisms: the heart drug digitalis from foxgloves, the painkiller aspirin from willow, and penicillin, which Alexander Fleming discovered from the Penicillium mould. Today the pharmaceutical industry synthesises most new drugs, though some still begin with a substance found in a plant.

1. Preclinical testing in a laboratory, using cells, tissues and live animals, checks toxicity, efficacy and dose.
2. Clinical trials use healthy volunteers and patients. They start with very low doses.
3. If the drug is safe, further trials find the optimum dose.
4. In double blind trials, some patients are given a placebo, and nobody involved knows who is getting the real drug.

Monoclonal antibodies (Biology, Higher tier)

Monoclonal antibodies come from a single clone of cells and bind to one specific antigen. Mouse lymphocytes that make the antibody are fused with a tumour cell to make a hybridoma, which can both divide and make the antibody; it is cloned to give large amounts. Uses include pregnancy tests, measuring hormones and other chemicals in blood, finding molecules in cells with a fluorescent dye, and carrying drugs or radioactive substances to cancer cells. They have caused more side effects than expected.

Plant diseases (Biology only)

• Signs (Higher tier): stunted growth, spots on leaves, rot, growths, malformed stems or leaves, discolouration, and pests.
• Identification (Higher tier): a gardening manual or website, a laboratory test, or a testing kit containing monoclonal antibodies.
• Ion deficiencies: nitrate deficiency causes stunted growth (nitrate is needed for protein synthesis); magnesium deficiency causes chlorosis (magnesium is needed for chlorophyll).
• Defences: physical (cellulose cell walls, a tough waxy cuticle, bark), chemical (antibacterial chemicals, poisons that deter herbivores) and mechanical (thorns and hairs, leaves that droop or curl when touched).

Exam questions with answers

Why can't antibiotics be used to treat measles?

Measles is caused by a virus. Antibiotics kill bacteria but cannot kill viruses, which live and reproduce inside the body's own cells.

Explain how a vaccine prevents a person catching a disease.

1. The vaccine contains a small amount of a dead or inactive form of the pathogen.
2. White blood cells respond by producing antibodies against it.
3. If the real pathogen enters the body later, the white blood cells recognise it and quickly produce the correct antibodies.
4. The pathogen is destroyed before it can cause illness.

Common mistakes

• Saying antibiotics kill viruses. They only kill bacteria.
• Saying vaccines contain antibodies. They contain dead or inactive pathogens; your white blood cells make the antibodies.
• Saying bacteria become resistant because of the antibiotic changing them. Resistant strains are ones that already survive the antibiotic and then multiply.
• Mixing up the pathogen types. Learn one disease from each: measles (virus), salmonella (bacterium), rose black spot (fungus), malaria (protist).
• Calling a placebo a fake drug given to everyone. Only some patients get it, so results can be compared.

Practise with a short timed paper

Infection and response is in Paper 1 of AQA GCSE Biology and Combined Science. The daily GCSE Biology Higher and GCSE Combined Science Higher papers are short timed papers of 8 questions, with a new one every day. The topic before it is organisation.

Read next

Biology · Disease and immunityAntibody structure: what antibodies are and how they workAntibody structure explained: the chains, variable regions and binding sites, how antibodies fight infection, and how monoclonal antibodies are made.