AQA GCSE Biology 4.3 Infection and response: a complete overview of pathogens, immunity and drugs

What module 4.3 actually demands

Infection and response is about the battle between pathogens and the body, and how medicine helps. The examiners reward precise vocabulary (pathogen, antigen, antibody, antitoxin), correct named examples, and clear reasoning about how vaccines, antibiotics and monoclonal antibodies work. Several parts are higher tier only, especially monoclonal antibodies.

This guide walks through the module and ties together the matching dot-point pages, each with its own practice questions.

Pathogens and communicable disease

A pathogen is a microorganism that causes a communicable disease. The four types are bacteria, viruses, fungi and protists. Learn AQA's named examples: measles, HIV and tobacco mosaic virus (viruses), salmonella and gonorrhoea (bacteria), rose black spot (fungus) and malaria (protist). Bacteria reproduce fast and make toxins; viruses reproduce inside cells and damage them.

How pathogens spread and how to stop them

Pathogens spread by direct contact, through water, in the air (droplets), and by vectors such as mosquitoes. Spread is reduced by hygiene, isolating infected people, destroying vectors and vaccination. In plants, growers remove infected plants and use chemical sprays.

Defending the body

The body's non-specific defences are the skin, nose, trachea and bronchi (mucus and cilia) and stomach acid. If pathogens get past these, the immune system responds: white blood cells carry out phagocytosis, produce specific antibodies, and produce antitoxins.

Vaccination, drugs and monoclonal antibodies

Vaccination introduces a dead or inactive pathogen so the body makes antibodies and memory cells, giving immunity. Antibiotics kill bacteria but not viruses; painkillers only treat symptoms. New drugs are tested for toxicity, efficacy and dose in preclinical and clinical trials, often using a placebo and a double-blind design.

Monoclonal antibodies (higher tier) are identical antibodies from a single clone, made via a hybridoma. They are highly specific and are used in pregnancy tests, diagnosis, research and targeted treatment.

How module 4.3 is examined

• Recall. Named pathogens and the diseases they cause, and the routes of spread.
• Explanation. How vaccines, antibodies, antitoxins and phagocytosis work.
• Application and data. Antibiotic-resistance data, vaccination uptake and disease graphs.
• Drug development. Stages of testing, placebos and double-blind trials.

Check your knowledge

A mix of recall and application questions covering module 4.3. Attempt them under timed conditions, then check against the solutions.

1. Name the four types of pathogen. (2 marks)
2. Give one named disease caused by a virus and one caused by a protist. (2 marks)
3. State two ways pathogens can be spread. (2 marks)
4. Describe three ways white blood cells defend the body. (3 marks)
5. Explain how vaccination produces immunity. (3 marks)
6. Explain why antibiotics cannot treat influenza. (1 mark)
7. State what a placebo is in a clinical trial. (1 mark)
8. Describe how a hybridoma cell is made. (2 marks)