6.3.3Global Challenges

White Blood Cells and Platelets — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions

5 min read 10 parts Lesson 11 of 18

📚 OCR Gateway GCSE Combined Science A J250 - Biology Hub / Topical Revision

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Part 1 of 8Components of blood

Blood is a tissue made of plasma, red blood cells, white blood cells and platelets. Plasma is the pale yellow liquid that carries dissolved substances such as glucose, urea, hormones and carbon dioxide, and distributes heat. Red blood cells carry oxygen. This lesson concentrates on white blood cells and platelets, which are part of the body's defence.

Part 2 of 8White blood cells

White blood cells (leucocytes) are larger than red cells, have a nucleus, and are far fewer in number. They are produced in bone marrow. The two main groups that you need to know are phagocytes and lymphocytes.

Type	Structure	How it protects
Phagocyte	Large, with a lobed or bean-shaped nucleus and a flexible cell membrane	Engulfs and digests pathogens by phagocytosis
Lymphocyte	Large round nucleus, little cytoplasm	Produces antibodies or antitoxins; some remain as memory cells

Phagocytosis happens in stages. The phagocyte is attracted to chemicals released by the pathogen, flows around it and engulfs it in a vacuole. Enzymes from lysosomes are released into the vacuole to digest and destroy the pathogen. Phagocytosis is non-specific: phagocytes respond to many types of pathogen.

Lymphocytes act in a more specific way. When a pathogen enters, a lymphocyte whose antibody fits the pathogen's antigen is stimulated to multiply and produce large numbers of the matching antibody. Some produce antitoxins that neutralise toxins released by bacteria.

Part 3 of 8Platelets

Platelets are small fragments of cells with no nucleus, produced in bone marrow.

Blood clotting

Their role is blood clotting.

When a blood vessel is damaged, platelets stick together at the site and release chemicals that trigger a series of reactions in the plasma.

Fibrin

A soluble protein called fibrinogen is converted to insoluble fibrin, which forms a mesh of fibres that traps red cells, making a clot.

The clot dries to form a scab.

Clotting prevents excessive loss of blood and also prevents pathogens from entering through the wound.

People with haemophilia lack a clotting factor and may bleed severely even from small cuts.

Part 4 of 8Observing blood

A stained blood smear can be examined under a light microscope.

Red cells appear as many small pink discs without a nucleus.

White cells are rarer and show purple-stained nuclei.

Platelets appear as tiny fragments.

When making a scientific drawing, use a sharp pencil, label the structures,

and add a magnification calculated by dividing the size of the drawing by the actual size.

High numbers of white blood cells in a blood test can suggest infection; lower numbers might indicate that the immune system is weakened,

as in HIV infection.

Leukaemia is a cancer in which too many abnormal white blood cells are made in the bone marrow.

Part 5 of 8Connections

These cells are the basis for the immune response, in which the number of antibodies rises when a pathogen first invades, and for vaccines, which prepare memory lymphocytes. They also link to the skin and other non-specific defences, which are the first line. Together these lessons show the layered nature of defence: barriers first, then phagocytes, then specific lymphocyte responses.

Part 6 of 8Magnification from a drawing

You may be asked to calculate magnification from a drawing or micrograph.

The formula is: magnification = size of image / actual size.

Be careful with units.

If a white blood cell has a diameter of 15 micrometres and your drawing shows it as 45 mm across, convert the 45 mm to micrometres (45 000) and divide: 45 000 / 15 = x3000.

A red blood cell is typically about 7 to 8 micrometres in diameter,
so you can estimate the size of a white blood cell by comparing it with the red cells next to it.

Since the white blood cell in a smear is about twice the width of a red cell, its diameter is about 15 micrometres.

Remember to rearrange the formula when you are given the magnification and the image size and are asked for the actual size: actual size = image size / magnification.

Part 7 of 8Antibodies and antitoxins in more detail

Each lymphocyte makes antibodies of one shape only.

The antibody's binding site is complementary to the antigen on the pathogen, in the same way that an enzyme's active site fits its substrate.

When the matching lymphocyte meets its antigen, it divides rapidly, so that many identical cells make the same antibody.

The antibodies bind to the pathogens, clumping them together so that phagocytes can engulf them more easily, or they mark them for destruction.

Antitoxins bind to toxins and neutralise them.

Memory cells

Some of the cloned lymphocytes become memory cells, which remain in the body for years and respond very quickly if the same antigen appears again.

This is the basis of immunity and vaccination, covered in the next lessons.

Part 8 of 8Exam-style questions

Describe how a phagocyte destroys a bacterium. [3]3 marks
Explain how platelets help to prevent infection when the skin is cut. [3]3 marks
A blood sample has 8 000 white blood cells per mm³. A patient with an infection has 15 000. Calculate the percentage increase. [2]2 marks

Key terms

Term	Meaning
Phagocyte	A white blood cell that engulfs and digests pathogens.
Lymphocyte	A white blood cell that makes antibodies.
Phagocytosis	Engulfing and digesting of a pathogen by a cell.
Antibody	A protein made by lymphocytes that binds to a specific antigen.
Antitoxin	A protein that neutralises a toxin.
Platelet	A cell fragment involved in blood clotting.
Fibrin	An insoluble protein that forms a mesh in a blood clot.
Antigen	A molecule on a pathogen that triggers an immune response.

Avoid these mistakes

Saying that lymphocytes engulf pathogens. That is the job of phagocytes.

Writing that platelets are whole cells with a nucleus; they are fragments.

Saying that fibrin is a liquid; the soluble fibrinogen becomes insoluble fibrin.

Confusing antibodies (made by lymphocytes) with antigens (on the pathogen).

Stating that red blood cells defend against disease.

Last updated: 20 October 2026