The Immune System
If pathogens get past the body's physical and chemical barriers, the immune system responds. White blood cells are the key players. In this lesson you will learn about the three ways white blood cells defend the body: phagocytosis, antibody production, and antitoxin production. You will also learn about antigens, memory cells, and immunity.
White Blood Cells (Leucocytes)
White blood cells are found in the blood and lymphatic system. They are part of the body's second and third lines of defence. There are several types, but for GCSE you need to know about two main groups:
| Type | Role | Specific or non-specific? |
|---|
| Phagocytes | Engulf and digest pathogens | Non-specific (attack any pathogen) |
| Lymphocytes | Produce antibodies and antitoxins | Specific (target particular pathogens) |
1. Phagocytosis
Phagocytosis is the process by which phagocytes (a type of white blood cell) engulf and destroy pathogens. It is a non-specific response — phagocytes will attack any foreign pathogen they detect.
The Process of Phagocytosis
- The phagocyte detects the pathogen (it recognises it as "foreign" because of the antigens on its surface)
- The phagocyte moves towards the pathogen (this movement is called chemotaxis)
- The phagocyte engulfs the pathogen by wrapping its cell membrane around it, forming a phagosome (a vesicle containing the pathogen)
- Lysosomes (organelles containing digestive enzymes) fuse with the phagosome
- The enzymes digest and destroy the pathogen
- The harmless remains are absorbed or expelled from the cell
Key Points
- Phagocytosis is fast — it begins within minutes of infection
- It is non-specific — phagocytes do not distinguish between different types of pathogen
- Phagocytes can also present fragments of the digested pathogen on their surface (antigen presentation), which helps activate lymphocytes
Exam tip: When describing phagocytosis in an exam, use the correct terminology: the phagocyte engulfs the pathogen (do not say "eats" or "swallows"). Then enzymes digest the pathogen. Mention that lysosomes provide the enzymes.
2. Antibody Production
Lymphocytes are white blood cells that produce antibodies. This is a specific immune response — it targets particular pathogens.
What Are Antigens?
- Antigens are unique proteins (or other molecules) found on the surface of cells and pathogens
- Every type of pathogen has its own unique set of antigens — like a molecular fingerprint
- Your own body cells also have antigens, but the immune system recognises these as "self" and does not attack them
- Foreign antigens (on pathogens) are recognised as "non-self" and trigger an immune response
How Antibodies Work
- A pathogen enters the body with specific antigens on its surface
- Lymphocytes detect the foreign antigens
- The lymphocyte with the correct complementary antibody shape is activated (each lymphocyte can only recognise one specific antigen)
- This lymphocyte divides rapidly (by mitosis) to produce many identical copies — a process called clonal expansion
- These lymphocyte clones produce large quantities of antibodies
- The antibodies are specific — they have a complementary shape to the antigen, like a lock and key
- Antibodies bind to the antigens on the pathogen surface
What Do Antibodies Do?
| Action | Explanation |
|---|
| Agglutination (clumping) | Antibodies bind to antigens on multiple pathogens, causing them to clump together. This makes it easier for phagocytes to find and engulf them. |
| Neutralisation | Antibodies can block the parts of the pathogen that are used to infect cells, preventing the pathogen from entering host cells |
| Marking for destruction | Antibodies act as "flags" that signal to phagocytes which cells to destroy (this is called opsonisation) |
Exam tip: Antibodies are specific — each type of antibody only binds to one specific type of antigen. This is because of the complementary shape of the antibody's binding site and the antigen. Use the phrase "lock and key" to describe this relationship in exams.
3. Antitoxin Production
- Some lymphocytes produce antitoxins instead of antibodies
- Antitoxins are molecules that neutralise (counteract) the toxins produced by bacteria
- Remember: bacteria cause disease by releasing toxins. Antitoxins bind to these toxins and make them harmless.
- This does not kill the bacteria directly, but it stops the toxins from causing damage
| Defence | What it targets | What it does |
|---|
| Antibodies | Antigens on pathogens | Bind to antigens → agglutination, neutralisation, opsonisation |
| Antitoxins | Toxins released by bacteria | Neutralise the toxins, preventing them from causing harm |
Memory Cells and Immunity
Primary Immune Response
- When a pathogen enters the body for the first time, the immune response is slow
- It takes time for the correct lymphocyte to be found, activated, and to multiply
- During this time, the person becomes ill because the pathogen is reproducing and causing damage
- Eventually, enough antibodies are produced to fight off the infection
Memory Cells
- After the infection is cleared, most of the lymphocytes die
- However, some remain as memory lymphocytes (memory cells)
- Memory cells persist in the body for many years, sometimes for life
- They "remember" the specific antigen of the pathogen
Secondary Immune Response
- If the same pathogen enters the body again, the memory cells recognise its antigens immediately
- The memory cells divide rapidly and produce antibodies much more quickly and in much larger quantities than during the primary response
- The pathogen is destroyed before it can cause symptoms
- The person is said to be immune to that disease
Comparing Primary and Secondary Responses
| Feature | Primary response | Secondary response |
|---|
| Speed | Slow (days to weeks) | Fast (hours to days) |
| Antibody levels | Low, build up gradually | High, produced rapidly |
| Symptoms? | Person usually becomes ill | Person usually does not become ill |
| Memory cells formed? | Yes | Yes (more are formed) |