The Immune System

The immune system is the body's specific defence against pathogens. While non-specific defences (skin, mucus, phagocytes) work against all pathogens, the immune system targets individual pathogens using specialised white blood cells called lymphocytes. This lesson explains how lymphocytes recognise and destroy pathogens, and how immunological memory provides long-term protection.

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Antigens — The Key to Specificity

Every pathogen has unique molecules on its surface called antigens. These antigens are like molecular fingerprints — they are different for every type of pathogen.

Term	Definition
Antigen	A unique protein on the surface of a pathogen that the immune system can recognise
Self antigen	A protein on the surface of your own cells — your immune system recognises these as "self" and does not attack them
Non-self antigen	A protein on the surface of a foreign cell or pathogen — triggers an immune response

The ability to distinguish between self and non-self is the foundation of the immune system. When the body detects non-self antigens, it knows a pathogen has entered and mounts a specific immune response.

Exam Tip: Antigens are the trigger for the entire immune response. If you are asked "How does the body recognise a pathogen?", the answer is always: the immune system detects non-self antigens on the pathogen's surface.

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White Blood Cells — The Defenders

There are three main types of white blood cell involved in the immune response. Each has a different role:

White Blood Cell Type	Role
Phagocytes	Engulf and digest pathogens (phagocytosis) — a non-specific response
Lymphocytes	Produce antibodies that are specific to the antigens on a particular pathogen
Memory cells	Remain in the blood after an infection; allow a faster response if the same pathogen returns

How Lymphocytes Work

Lymphocytes are the white blood cells responsible for the specific immune response. There are billions of different lymphocytes in your body, each producing a different type of antibody. Here is how the process works:

graph TD
    A[Pathogen enters the body] --> B[Pathogen has unique antigens on its surface]
    B --> C[Phagocytes engulf pathogen and present antigens]
    C --> D[Correct lymphocyte recognises the antigen]
    D --> E[Lymphocyte divides rapidly - clonal expansion]
    E --> F[Large number of identical lymphocytes produced]
    F --> G[Lymphocytes produce specific antibodies]
    G --> H[Antibodies bind to antigens on pathogen]
    H --> I[Pathogen is neutralised, clumped, or destroyed]
    E --> J[Some lymphocytes become memory cells]
    J --> K[Memory cells remain in blood for years]

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Antibodies — How They Work

Antibodies are Y-shaped proteins produced by lymphocytes. Each antibody has a specific shape that fits one particular antigen — like a lock and key.

What Antibodies Do

Antibodies help destroy pathogens in several ways:

Antibody Action	Description
Neutralisation	Antibodies bind to the pathogen's surface, preventing it from entering or damaging cells
Agglutination (clumping)	Antibodies cause pathogens to clump together, making it easier for phagocytes to engulf them
Toxin neutralisation	Antibodies bind to toxins produced by bacteria, making the toxins harmless
Complement activation	Antibodies trigger a cascade of proteins that punch holes in the pathogen's membrane

Key Points About Antibodies

• Each antibody is specific to one particular antigen
• Antibodies are produced by lymphocytes (B lymphocytes specifically)
• It takes time to produce enough antibodies during a first infection — this is why you feel ill
• On second exposure, memory cells enable antibody production to be much faster

Exam Tip: Antibodies do NOT directly kill pathogens. They bind to antigens and then either neutralise the pathogen, clump pathogens together for phagocytes to destroy, or mark pathogens for destruction. Make sure you describe the mechanism correctly.

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The Primary and Secondary Immune Response

The speed at which the body responds to a pathogen depends on whether it has encountered that pathogen before:

Primary Immune Response (First Infection)

1. The pathogen enters the body for the first time
2. The body has never seen these antigens before
3. It takes time to find the correct lymphocyte and produce enough antibodies
4. During this delay, the pathogen reproduces and the person feels ill
5. Eventually, enough antibodies are produced to overcome the infection
6. Memory cells are produced and remain in the blood

Secondary Immune Response (Second Infection)

1. The same pathogen enters the body again
2. Memory cells recognise the pathogen's antigens immediately
3. Memory cells divide rapidly and produce antibodies much faster and in much larger quantities
4. The pathogen is destroyed before it can cause symptoms
5. The person does not become ill — they are immune

Feature	Primary Response	Secondary Response
Speed	Slow (days to weeks)	Fast (hours to days)
Antibody quantity	Lower	Much higher
Symptoms	Person feels ill	Person does not feel ill
Memory cells	Created during this response	Already present from first exposure

Exam Tip: A very common exam question shows a graph of antibody levels over time, with a first and second exposure to the same pathogen. You must be able to explain why the secondary response is faster and produces more antibodies — the answer is memory cells.

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Antitoxins

Some white blood cells produce antitoxins rather than antibodies. These are chemicals that neutralise toxins produced by bacterial pathogens.

• Antitoxins bind to the specific toxin and make it harmless
• They do not kill the bacteria directly but counteract the damage caused by the toxins
• This is particularly important for diseases like tetanus, where the toxin causes the main symptoms

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The Three Roles of White Blood Cells — Summary

For the AQA exam, you must remember that white blood cells defend the body in three main ways: