Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

9.1.14Transport in Animals

White Blood Cells

5 min read 10 parts Lesson 15 of 15 1 quiz

0%read

Section 1 of 10Components of blood

Blood cells
Blood Clotting
Blood Clotting
Red Blood Cells & Osmosis
Control of Blood Glucose
blood clotting pathway
Blood cells
Blood cells
Blood composition: plasma, red cells, white cells and platelets
Components of blood and their proportions
Components of blood and their proportions
Red blood cell structure and size
Red blood cell structure and size
Platelets and fibrin mesh in blood clotting
Platelets and fibrin mesh in blood clotting

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.

Section 2 of 10White blood cells

Lymphocyte
Three stages of phagocytosis of bacteria by a phagocyte
pasted 1782294568426 0
Blood components table (red cells, white cells, platelets, plasma)
Monocyte
pasted 1782305713593 0
pasted 1782305704867 0
pasted 1782294545200 0
A phagocyte engulfing and digesting a pathogen
A phagocyte engulfing and digesting a pathogen

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.

TypeStructureHow it protects
PhagocyteLarge, with a lobed or bean-shaped nucleus and a flexible cell membraneEngulfs and digests pathogens by phagocytosis
LymphocyteLarge round nucleus, little cytoplasmProduces 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.

Section 3 of 10Platelets

Blood Vessels
Blood Vessels
Blood Vessel Types
Blood Vessels
Three stages of blood clotting at a broken vessel
Blood clotting steps (platelets, fibrin threads)
Blood vessel injury and clot formation

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.

Section 4 of 10Observing blood

Blood smear: red blood cells, white blood cells, platelet

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

1
Red cells appear as many small pink discs without a nucleus.
2
White cells are rarer and show purple-stained nuclei.
3
Platelets appear as tiny fragments.
4
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.
5
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.
6
Leukaemia is a cancer in which too many abnormal white blood cells are made in the bone marrow.

Section 5 of 10Connections

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.

Section 6 of 10Magnification from a drawing

Magnification Formula

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

1
The formula is: magnification = size of image / actual size.
2
Be careful with units.
3
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.
4
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.

Section 7 of 10Antibodies and antitoxins in more detail

Matching lymphocyte makes specific antibody for antigen
Lymphocytes make antibodies specific to antigens
pasted 1782549810983 0
Antibody specificity: only the matching lymphocyte responds to antigen
Vaccination produces long-term immunity (memory cells)

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.

Section 8 of 10Exam-style questions

Red blood cells, white blood cells and platelets
Antibodies and lymphocytes (specific antibody to antigen)
Lymphocyte makes antibodies specific to antigen
antibody responses landscape

  1. Describe how a phagocyte destroys a bacterium. [3]3 marks

  2. Explain how platelets help to prevent infection when the skin is cut. [3]3 marks

  3. 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

Section 9 of 10Key terms

TermMeaning
PhagocyteA white blood cell that engulfs and digests pathogens.
LymphocyteA white blood cell that makes antibodies.
PhagocytosisEngulfing and digesting of a pathogen by a cell.
AntibodyA protein made by lymphocytes that binds to a specific antigen.
AntitoxinA protein that neutralises a toxin.
PlateletA cell fragment involved in blood clotting.
FibrinAn insoluble protein that forms a mesh in a blood clot.
AntigenA molecule on a pathogen that triggers an immune response.

Section 10 of 10Exam technique

1
Saying that lymphocytes engulf pathogens. That is the job of phagocytes.
2
Writing that platelets are whole cells with a nucleus; they are fragments.
3
Saying that fibrin is a liquid; the soluble fibrinogen becomes insoluble fibrin.
4
Confusing antibodies (made by lymphocytes) with antigens (on the pathogen).
5
Stating that red blood cells defend against disease.
Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A  ⇔  A = I / M  ⇔  I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.