The Immune Response
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of the roles of pathogens and how they cause infectious disease.
What is a pathogen?
A pathogen is a microorganism that causes disease, such as a virus, bacterium, fungus, or protist
How do bacteria and viruses cause illness?
Bacteria produce toxins that damage cells and tissues, while viruses reproduce inside cells and cause them to burst, destroying them
How are pathogens destroyed in the stomach?
By hydrochloric acid that the stomach produces
Topic Explainer Video
Check out this @JoeDoesBiology video that explains the immune response, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!
The Immune Response
The Role of the Immune System
- The immune system defends the body against pathogens that cause disease.
- If a pathogen enters the body, the immune system tries to identify and destroy it.
- The immune system is made of your white blood cells, they identify pathogens that are in the blood.
Types of White Blood Cell Defence
1. Phagocytosis
- A type of white blood cell (called a phagocyte) will surround, engulf, and digest pathogens.
- This is a non-specific response (phagocytes target all pathogens).
- The process:
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- Detect and engulf the pathogen.
- Enclose pathogen in a vacuole.
- Digest pathogen using enzymes.
Diagram showing phagocytocis of a bacterium.
2. Antibody Production
- All cells (including all pathogens) have protein markers on their surface called antigens.
- Antigens are specific for each cell and are used by the immune system to recognise which cells are your own cells and which cells may be pathogens or foreign cells.
- White blood cells called lymphocytes produce antibodies which are specific and complementary to the antigens on a pathogen.
- The antibodies bind to antigens and mark the pathogen for destruction by phagocytes.
- After recovery, memory cells remain to respond faster on re-infection - this gives us immunity.
Memory cells will recognise the foreign antigen upon reinfection and antibodies will be produced much faster to destroy the pathogen,
3. Antitoxin Production
- Some white blood cells produce antitoxins.
- Antitoxins:
- Neutralise toxins released by bacteria.
- Prevent toxins from damaging cells.
More about Memory Cells and Long-Term Immunity
- After an infection, some white blood cells become memory cells.
- Memory cells:
- Remain in the blood for a long time.
- Recognise antigens and respond quickly if the same pathogen enters again.
- Produce large quantities of the correct antibody rapidly and for longer.
- This is why you often don’t get ill twice from the same infection (e.g. chicken pox).
- This is also how vaccines work: they trigger the production of memory cells without causing disease.
Graph showing the primary immune response (the first infection with the pathogen) and the secondary immune response (second infection with the pathogen).
Key Terms
- Pathogen – A microorganism that causes disease.
- Antigen – A molecule on the surface of a pathogen that triggers an immune response.
- Antibody – A protein made by white blood cells that binds to a specific antigen.
- Phagocytosis – The process by which white blood cells engulf and digest pathogens.
- Antitoxin – A chemical that neutralises toxins produced by pathogens.
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Exam Tips
- Phagocytosis is non-specific, whereas antibody production is specific, the correct antibody is needed for the specific antigen on the surface of the pathogen.
- Don’t confuse antitoxins and antibodies (produced by white blood cells) with antibiotics (medicines that kill bacteria).
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Practice Question
Describe how white blood cells defend the body against pathogens. [4 marks]
Model Answer:
- Some white blood cells engulf and digest pathogens by phagocytosis.
- Other white blood cells produce antibodies that bind to specific antigens on pathogens to destroy the pathogen.
- Some white blood cells produce antitoxins to neutralise toxins made by bacteria.
- Some white blood cells become memory cells which respond faster and produce more antibodies when they encounter the pathogen a second time.
More Practice
Try to answer these practice questions from the TikTok videos on your own, then watch the videos to see how well you did!