Human defences and immunity — Edexcel GCSE Biology
Barriers and chemical defences, the specific immune response, how immunisation works, and the pros and cons of vaccination including herd immunity.
Revision notes
4 short notes, in the order of the specification. Each one 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 specific immune system responds to one particular pathogen. Antigens on the pathogen trigger lymphocytes to produce antibodies, which bind to the antigens and help destroy it. Memory lymphocytes are also made. If the same pathogen returns, they give a faster secondary response with more antibodies, so the person does not become ill.
Immunisation gives protection from a disease without having to catch it. An inactive form of the pathogen, such as a dead or weakened one, is usually injected. Its antigens trigger the production of antibodies and memory lymphocytes. If the live pathogen enters later, the memory lymphocytes make large numbers of antibodies quickly, so the person does not become ill.
Advantages and disadvantages of immunisation
Triple onlyImmunisation protects people from serious diseases, reduces the spread of disease and can prevent epidemics. Through herd immunity, when a large proportion of a population is immune, it also protects people who cannot be vaccinated. Disadvantages are that some people have side effects, it does not always work, boosters may be needed and some pathogens change often.
Exam questions
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Quick check questions
Name two physical barriers in the human body.
Show answer
Any two of: skin, mucus, cilia.
What do lysozymes do?
Show answer
They are enzymes that break down the cell walls of bacteria.
What is an antigen?
Show answer
A molecule, usually on the surface of a pathogen, that triggers an immune response.
Why is the secondary response better than the primary response?
Show answer
Memory lymphocytes make it faster and produce more antibodies.
Triple only: what is herd immunity?
Show answer
When a large proportion of the population is immune, so the pathogen cannot spread easily and non-immune people are protected.
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.
What are the body's physical barriers against pathogens?
The body's physical barriers are the skin, mucus and cilia. The skin forms a barrier that pathogens cannot easily pass through. Mucus in the nose and airways is sticky and traps pathogens, and cilia on the cells lining the airways move the trapped mucus away from the lungs. The cilia do not trap pathogens themselves.
Why is herd immunity important?
Triple only Herd immunity is important because it protects people who are not immune, such as babies too young to be vaccinated or people with a weak immune system. When a large proportion of a population is immune, the pathogen finds it hard to spread. If fewer people are vaccinated, herd immunity is lost and the disease can spread again.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026