Blood and the immune system — Edexcel International GCSE Biology
The components of blood, how red blood cells carry oxygen, how white blood cells defend the body, vaccination and clotting.
Revision notes
4 short notes, in the order of the specification. Each one in short:
Blood is a tissue made of plasma, red blood cells, white blood cells and platelets. Plasma transports carbon dioxide, digested food, urea, hormones and heat energy. Red blood cells transport oxygen: their biconcave disc shape gives a large surface area, they contain haemoglobin that combines with oxygen to form oxyhaemoglobin, and they have no nucleus.
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.
Vaccination and memory cells
Biology onlyVaccination is giving a person a harmless form of a pathogen, such as a dead or weakened one, that still carries its antigens. The antigens trigger lymphocytes to release antibodies and make memory cells. If the live pathogen enters later, memory cells recognise it, so antibody production is sooner, faster and in greater quantity.
Platelets and blood clotting
Biology onlyBlood clotting is how the blood seals a cut, preventing blood loss and stopping micro-organisms entering the body. Platelets collect at the wound and release chemicals that change the soluble plasma protein fibrinogen into insoluble fibrin. Fibrin forms a mesh of fibres that traps red blood cells to make a clot, which dries to form a scab.
Exam questions
7 exam-style questions (24 marks), each with its mark scheme.
Answer the questionsFlashcards
13 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of structure and functions in living organisms on one page, so you can see where this subtopic fits.
Open the mind mapPrint it
Free PDFs to print or save.
Quick check questions
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.
Biology only What do memory cells allow after vaccination?
Show answer
Antibody production that is sooner, faster and in greater quantity if the pathogen enters again.
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.
How do vaccines work?
Biology only Vaccines work by giving a harmless form of a pathogen, such as a dead or weakened one, that still has its antigens. Lymphocytes release antibodies and memory cells are made, without the person becoming ill. If the live pathogen enters later, memory cells recognise it, so antibodies are made sooner, faster and in greater quantity.
How do platelets help blood to clot?
Biology only Platelets collect at a cut and release chemicals that change the soluble plasma protein fibrinogen into insoluble fibrin. Fibrin forms a mesh of fibres that traps red blood cells, making a clot that dries into a scab. Clotting prevents blood loss and stops micro-organisms from entering the body through the wound.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026