Physical barriers and chemical defencesSpec 5.12
In short
The body's first defences against pathogens are physical barriers and chemical defences. The skin is a barrier, mucus in the nose and airways traps pathogens, and cilia move the mucus away from the lungs. Lysozymes in tears break down the cell walls of bacteria, and hydrochloric acid in the stomach kills swallowed pathogens.
The body has first lines of defence that try to stop pathogens getting in or kill them before they can cause harm.
Human physical barriers and chemical defences| Defence | Type | How it protects |
|---|
| Skin | Physical barrier | Forms a barrier that pathogens cannot easily pass through |
|---|
| Mucus | Physical barrier | A sticky liquid in the nose and airways that traps pathogens |
|---|
| Cilia | Physical barrier | Tiny hairs on the cells lining the airways that move the trapped mucus up and away from the lungs |
|---|
| Lysozymes | Chemical defence | Enzymes in tears and other secretions that break down the cell walls of bacteria |
|---|
| Hydrochloric acid | Chemical defence | Acid in the stomach that kills pathogens swallowed with food and drink |
|---|
Common mistake:Cilia do not trap pathogens; mucus does. Cilia move the mucus.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026
Frequently asked questions
How do vaccines work?
Vaccines work by putting an inactive form of a pathogen, such as a dead or weakened one, into the body. Its antigens trigger an immune response, so antibodies and memory lymphocytes are made. If the live pathogen enters later, memory lymphocytes carry out a fast secondary response with many antibodies, so the person does not become ill.
How do white blood cells fight infection?
White blood cells called lymphocytes fight infection by producing antibodies. Antigens on a pathogen trigger the lymphocytes with antibodies that fit to divide, and antibodies are produced. The antibodies bind to the antigens and help destroy the pathogen. Memory lymphocytes are also made, so the response is faster if the pathogen returns.
What is the difference between the primary and secondary immune response?
The primary response happens the first time a pathogen enters the body, and the person usually becomes ill while antibodies are made. In the secondary response, memory lymphocytes recognise the same antigens and respond faster, making more antibodies, so the pathogen is destroyed before it can make the person ill.
All 5 questions on Human defences and immunity
