Immunisation

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ImmunisationSpec 5.14

In short

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.

Immunisation gives protection from a disease without having to catch it. A person is given an inactive form of the pathogen, for example a dead or weakened one, usually by injection.

The inactive pathogen is put into the body. It cannot cause the disease, but its antigens are still present.
The antigens trigger an immune response and antibodies are made.
Memory lymphocytes are produced.
If the live pathogen enters later, the memory lymphocytes carry out a fast secondary response. Large numbers of antibodies are made quickly and the person does not become ill.

Immunisation and the immune response
	Natural infection	After immunisation
Pathogen present	Live pathogen	Inactive pathogen, which cannot cause the disease
Memory lymphocytes made?	Yes	Yes
Person becomes ill?	Yes, during the first (primary) response	No

Doctors sometimes give a second injection, called a booster, to increase the number of memory lymphocytes made.

Common mistake:

Immunisation does not put antibodies into the body. It makes your own body produce antibodies and memory lymphocytes.

Quick check

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.

Written and checked against the Edexcel GCSE Combined Science (1SC0) 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.

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