Human defence systems — AQA GCSE Biology
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Human defence systems explained
Non-specific defences stop pathogens entering the body or trap and destroy them before they cause infection.
Read the full explanation
They are not tailored to one pathogen. The skin is a physical barrier: its outer layer of dead cells is tough, and glands secrete antimicrobial substances. The nose has hairs and sticky mucus that trap particles and microorganisms in inhaled air. In the trachea and bronchi, ciliated epithelial cells beat mucus, with trapped pathogens, upwards towards the throat to be swallowed. The stomach contains hydrochloric acid, which is very acidic and kills many pathogens that are swallowed in food, drink or mucus. Together these defences reduce the chance of pathogens reaching tissues where they could multiply.
Students should be able to explain the role of the immune system in the defence against disease.
If a pathogen gets past the non-specific defences, the immune system responds specifically to that pathogen. White blood cells, mainly lymphocytes, recognise antigens on the pathogen surface. Each lymphocyte is specific to one antigen. When it binds, it divides to produce clones, some of which secrete antibodies that lock onto antigens, clumping pathogens so they can be destroyed by phagocytes. Other white blood cells, phagocytes, engulf and digest pathogens. Some lymphocytes remain as memory cells, so a second infection by the same pathogen triggers a faster, larger antibody response, often preventing illness. This is also how vaccination works. The immune response therefore destroys pathogens and provides long-lasting immunity.
If a pathogen enters the body the immune system tries to destroy the pathogen.
Antitoxins neutralise toxins produced by bacteria, not the pathogens themselves. A pathogen is a microorganism that causes disease, such as a bacterium, virus, fungus, or protist. The body has non-specific defences that try to stop pathogens entering, including the skin, mucus, cilia, and stomach acid. If a pathogen gets past these barriers, the immune system responds. White blood cells recognise the pathogen as foreign, usually by its antigens, and act to destroy it. For example, phagocytes can engulf pathogens, while lymphocytes produce antibodies that stick to antigens and help destroy the pathogen. The response is specific, and memory cells can remain so a later infection is fought faster.
White blood cells help to defend against pathogens by: • phagocytosis • antibody production • antitoxin production.
White blood cells are part of the immune system and defend the body in three main ways. In phagocytosis, a white blood cell engulfs a pathogen and digests it. In antibody production, lymphocytes produce antibodies that bind to antigens on a specific pathogen; this can clump pathogens together, neutralise them or help other white blood cells destroy them. In antitoxin production, white blood cells produce antitoxins that neutralise toxins released by pathogens, such as the toxins produced by some bacteria. Each response helps destroy or disable the pathogen. For example, antibodies against measles virus bind to measles antigens, while antitoxins can neutralise tetanus toxin. Memory cells formed during a response allow a faster second response.
Your focus
- Describe how the skin, nose, trachea and bronchi, and stomach defend the body.
- Explain how each defence prevents pathogens from causing infection.
- Distinguish non-specific defences from specific immune responses.
Show all 12 objectives
- Explain how white blood cells recognise and respond to pathogens.
- Describe how antibodies and phagocytes destroy pathogens.
- Explain how memory cells provide immunity after infection or vaccination.
- Define pathogen and give examples of types of pathogen.
- Describe how the body prevents pathogens from entering.
- Explain how the immune system responds once a pathogen has entered the body.
- Describe phagocytosis by white blood cells.
- Explain how antibodies are specific to antigens and help destroy pathogens.
- State that antitoxins neutralise toxins produced by pathogens.
Human defence systems exam tips
Marking Points
- Non-specific defences act against many pathogens and are not targeted at one specific pathogen.
- The skin acts as a physical barrier and produces antimicrobial secretions.
- The nose traps pathogens using hairs and sticky mucus.
- The trachea and bronchi are lined with cilia that move mucus containing pathogens away from the lungs.
- The stomach produces hydrochloric acid, which kills many swallowed pathogens.
- Mucus and cilia work together to remove pathogens from the airways.
- White blood cells recognise antigens on the surface of pathogens.
- Lymphocytes produce antibodies that are specific to the pathogen's antigens.
- Antibodies bind to antigens, causing pathogens to clump so phagocytes can engulf them.
- Phagocytes engulf and digest pathogens by phagocytosis.
- Memory cells remain after infection and give a faster, stronger response on re-infection.
- The immune response is specific to the pathogen and can provide immunity.
- Antitoxins neutralise toxins produced by bacteria, not the pathogens themselves.
- A pathogen is a microorganism that causes disease, for example a bacterium or virus.
- The body has non-specific barriers such as skin, mucus and stomach acid that reduce pathogen entry.
- If a pathogen enters the body, white blood cells recognise it as foreign, often through antigens on its surface.
- Phagocytosis and antibody production are ways white blood cells destroy pathogens.
- Phagocytosis involves a white blood cell engulfing and digesting a pathogen.
- Antibodies are proteins produced by lymphocytes that bind to specific antigens on a pathogen.
- Antibody binding can clump pathogens, neutralise them or help phagocytes destroy them.
- Antitoxins are produced by white blood cells and neutralise toxins released by pathogens.
- The response is specific because antibodies and memory cells recognise particular antigens.
- Memory cells remain after an infection and give a faster, stronger response on re-infection.
Examiner Tips
- 💡Name each defence and state exactly how it stops pathogens, using the correct structure.
- 💡Use the words mucus and cilia accurately and link them to the airways.
- 💡For the stomach, name hydrochloric acid rather than just saying acid.
- 💡Use the terms antigen, antibody, lymphocyte and phagocyte accurately.
- 💡Explain the sequence: recognition, antibody production, clumping, engulfing, memory.
- 💡Link memory cells to immunity and vaccination to show deeper understanding.
- 💡Use the term pathogen correctly and give a named example, such as a bacterium or virus, to show understanding.
- 💡Link the entry of a pathogen to a specific defence, such as phagocytosis or antibody production, rather than saying the body 'fights germs'.
- 💡Name the white blood cell process clearly, for example 'phagocytosis' or 'antibody production', rather than describing it vaguely.
- 💡Use the words antigen and antibody correctly: antigens are on the pathogen surface, antibodies are produced by lymphocytes and bind to antigens.
- 💡When describing antitoxins, state that they neutralise toxins, not that they kill pathogens directly.
Common Mistakes
- Calling these defences specific to one pathogen; the correction is that they are non-specific and act against many pathogens.
- Saying cilia trap pathogens; the correction is that mucus traps pathogens and cilia move the mucus.
- Saying the stomach produces enzymes that kill pathogens; the correction is that hydrochloric acid kills them.
- Saying antibodies kill pathogens directly; the correction is that antibodies bind antigens and help phagocytes destroy the pathogen.
- Saying all white blood cells produce antibodies; the correction is that lymphocytes produce antibodies while phagocytes engulf pathogens.
- Confusing antigens with antibodies; the correction is that antigens are on the pathogen surface and antibodies are produced by lymphocytes.
- Stating antitoxins destroy pathogens; correction: antitoxins neutralise toxins produced by bacteria, not the pathogens themselves.
- Thinking all microorganisms are pathogens; correction: only some microorganisms cause disease, and many are harmless or useful.
- Believing the skin is part of the immune system's internal response; correction: the skin is a non-specific barrier that prevents entry.
- Confusing antibodies with antitoxins; correction: antibodies bind to antigens on pathogens, while antitoxins neutralise toxins produced by pathogens.
- Thinking phagocytosis is specific to one pathogen; correction: phagocytosis is a general engulfing response, whereas antibody production is specific to antigens.
- Saying white blood cells 'eat' toxins; correction: antitoxins neutralise toxins, while phagocytes engulf pathogens.