White blood cells and the immune response

Structure and functions in living organisms · Blood and the immune system · note 2 of 2

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White blood cells and the immune responseSpec 2.62

In short

The immune response is the defence of the body against pathogens by white blood cells. Phagocytes ingest pathogens and destroy them, a process called phagocytosis. Lymphocytes recognise the antigens on a pathogen's surface and release antibodies. Each antibody is specific: its shape fits the antigens on one pathogen only, and it helps to destroy that pathogen.

The immune system protects the body from disease. White blood cells are the main part of this defence. Two types do different jobs: phagocytes and lymphocytes.

Pathogen: A microorganism that causes disease.
Antigen: A molecule on the surface of a pathogen that the immune system recognises as foreign.
Antibody: A protein released by a lymphocyte that is specific to one antigen and helps to destroy the pathogen.

Phagocytes

A phagocyte detects a pathogen.
It surrounds the pathogen and ingests (takes in) it. This is called phagocytosis.
The pathogen is then destroyed inside the phagocyte.

Lymphocytes

Lymphocytes recognise the antigens on the surface of a pathogen.
They release antibodies into the blood.
Each antibody is specific to one pathogen. Its shape fits the antigens on that pathogen only.
The antibodies attach to the pathogen and help to destroy it.

Top: a phagocyte detects a pathogen, surrounds it and ingests it. Bottom: a lymphocyte releases Y-shaped antibodies, which bind to matching antigens on the surface of a pathogen, while an antibody with a different shape does not fit. — Phagocytes **ingest** pathogens. Lymphocytes release **antibodies** that fit only the matching antigens.

Exam tip:

Antibodies are specific. Say that the antibody has a shape that fits the antigen on that pathogen, and that a different pathogen needs a different antibody.

Quick check

Name the four components of blood.
Show answer
Red blood cells, white blood cells, platelets and plasma.
Give three adaptations of a red blood cell for carrying oxygen.
Show answer
Biconcave shape, no nucleus, contains haemoglobin.
How do phagocytes destroy pathogens?
Show answer
They ingest (engulf) them.
What do lymphocytes release?
Show answer
Antibodies that are specific to the pathogen.

Written and checked against the Edexcel IGCSE Science Double Award (4SD0) specification · Updated October 2026

Frequently asked questions

How are red blood cells adapted to carry oxygen?

Red blood cells are adapted to carry oxygen in three ways. Their biconcave disc shape gives a large surface area for oxygen to diffuse in and out quickly. They contain haemoglobin, which combines with oxygen in the lungs to form oxyhaemoglobin and releases it in the tissues. They have no nucleus, leaving more room for haemoglobin.

How do white blood cells fight infection?

White blood cells fight infection in two ways. Phagocytes surround pathogens and ingest them, then destroy them inside the cell, which is called phagocytosis. Lymphocytes recognise the antigens on a pathogen's surface and release antibodies that are specific to that pathogen. The antibodies attach to the pathogen and help to destroy it.

Why are antibodies specific?

Antibodies are specific because each antibody has a shape that fits the antigens on one particular pathogen only. Antigens are molecules on the surface of a pathogen that the immune system recognises as foreign. A different pathogen has different antigens, so lymphocytes must release a different antibody to help destroy it.

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