Overview of Body Defences
The body has three lines of defence against pathogens: primary defences (physical and chemical barriers), the immune system (white blood cells), and specific immune responses (antibodies and antitoxins).
Your body is constantly under attack from pathogens in the air, in food, and on surfaces. To protect you, it has a series of defence mechanisms that work together to stop pathogens from entering and to destroy any that do get in.
The Immune System — White Blood Cells
If pathogens get past the primary defences, the immune system responds. White blood cells are the key cells of the immune system. They defend the body in three main ways: phagocytosis, producing antibodies, and producing antitoxins.
Unlike primary defences, the immune system's response is specific — it can target particular pathogens. White blood cells are part of the blood and are constantly circulating, patrolling for pathogens.
1. Phagocytosis
Phagocytosis is the process by which phagocyte white blood cells engulf and digest pathogens. This is a non-specific defence — phagocytes can destroy any pathogen they encounter.
- A phagocyte (a type of white blood cell) recognises a pathogen as foreign
- The phagocyte changes shape to surround and engulf the pathogen
- The pathogen is enclosed in a vesicle (phagosome) inside the phagocyte
- Enzymes inside the phagocyte digest and destroy the pathogen
Phagocytosis is a non-specific defence because the same phagocyte can engulf any type of pathogen — it does not need to recognise a specific antigen.
2. Producing Antibodies
Antibodies are proteins produced by lymphocyte white blood cells. Each antibody is specific to one type of pathogen — it locks onto the pathogen's antigens in a lock-and-key mechanism.
- Every pathogen has unique molecules (antigens) on its surface
- When a lymphocyte encounters a pathogen with antigens it does not recognise, it starts to produce antibodies that are complementary in shape to those specific antigens
- The antibodies lock onto the antigens (lock-and-key mechanism) — each antibody is specific to one type of antigen
- Antibodies bind to and neutralise pathogens: they can clump pathogens together (agglutination) so phagocytes can engulf them more easily, or they can bind to toxins to neutralise them
- After the infection, memory cells (a type of lymphocyte) remain in the blood. If the same pathogen enters the body again, memory cells produce the correct antibodies much faster and in greater numbers, destroying the pathogen before you feel ill — this is natural immunity
Key principle: Antibodies are specific — each antibody only fits one type of antigen (lock-and-key). A different antibody is needed for every different pathogen.
3. Producing Antitoxins
Antitoxins are proteins produced by white blood cells that neutralise toxins produced by pathogens. They bind to the toxins and prevent them from damaging body cells.
- Some bacteria cause disease by producing toxins (poisonous substances) that damage tissues
- White blood cells produce antitoxins that are specific to each toxin
- The antitoxin binds to the toxin and neutralises it — preventing the toxin from harming body cells
- This is different from antibodies: antibodies target the pathogen itself; antitoxins target the toxins produced by the pathogen
| Defence Mechanism |
Cell Type |
Specific or Non-specific? |
What It Targets |
| Phagocytosis |
Phagocytes |
Non-specific |
Any pathogen the phagocyte encounters |
| Antibodies |
Lymphocytes |
Specific |
Antigens on a specific pathogen |
| Antitoxins |
Lymphocytes |
Specific |
Toxins produced by a specific pathogen |
Worked Examples
Worked Example 1: Explaining How the Skin Prevents Infection
Question: Explain how the skin acts as a primary defence against pathogens. [3 marks]
Solution:
- The skin is a physical barrier that covers the entire body surface, preventing pathogens from reaching the tissues beneath (1 mark)
- If the skin is cut, blood clotting quickly seals the wound to prevent pathogens entering through the break (1 mark)
- The skin also produces antimicrobial secretions (such as sebum and sweat containing lysozyme) that kill bacteria on the skin surface (1 mark)
Worked Example 2: Describing Phagocytosis
Question: Describe the process of phagocytosis. [4 marks]
Solution:
- A phagocyte (a type of white blood cell) recognises a pathogen as foreign to the body (1 mark)
- The phagocyte changes shape and surrounds the pathogen, engulfing it (1 mark)
- The pathogen is enclosed inside a vesicle (phagosome) within the phagocyte (1 mark)
- Digestive enzymes inside the phagocyte break down and destroy the pathogen (1 mark)
Worked Example 3: Explaining the Lock-and-Key Mechanism of Antibodies
Question: A new virus enters the body. Explain how white blood cells produce antibodies to destroy this virus. [4 marks]
Solution:
- The virus has specific antigens (molecules) on its surface that are unique to that virus (1 mark)
- A lymphocyte (white blood cell) encounters the virus and produces antibodies with a shape that is complementary to the virus's antigens — this is the lock-and-key mechanism (1 mark)
- The antibodies bind to the antigens on the virus, which can cause the viruses to clump together (agglutination) making it easier for phagocytes to engulf and destroy them (1 mark)
- After the infection, memory cells remain in the blood. If the same virus enters again, these memory cells rapidly produce the correct antibodies in greater quantities, destroying the virus before it causes illness (1 mark)
Worked Example 4: Distinguishing Between Antibodies and Antitoxins
Question: A student says "Antibodies and antitoxins do the same thing." Is the student correct? Explain your answer. [3 marks]
Solution:
The student is incorrect.
- Antibodies bind to antigens on the surface of pathogens themselves, targeting and neutralising the pathogen directly or marking it for destruction by phagocytosis (1 mark)
- Antitoxins bind to and neutralise toxins (poisonous chemicals) that are produced by pathogens — they target the harmful chemicals, not the pathogen itself (1 mark)
- Although both are specific proteins produced by white blood cells, they target different things: antibodies target the pathogen, antitoxins target the pathogen's toxins (1 mark)
Worked Example 5: Applying Knowledge to an Unfamiliar Scenario
Question: A person has a condition that prevents their stomach from producing acid. Explain why this person may be more likely to suffer from food poisoning than someone with a healthy stomach. [3 marks]
Solution:
- Stomach acid (hydrochloric acid) normally kills bacteria that are present in food, preventing them from reaching the intestines where they could cause disease (1 mark)
- Without stomach acid, bacteria in food are not killed and can survive passage through the stomach (1 mark)
- These surviving bacteria can then reach the intestines, reproduce, produce toxins, and cause food poisoning — meaning this person is more likely to get ill from contaminated food (1 mark)
Answers
A1. Any three from:
- Skin — acts as a physical barrier covering the body surface, preventing pathogens from entering tissues beneath; also produces antimicrobial secretions that kill bacteria (2 marks)
- Mucus membranes + cilia — mucus traps pathogens in the respiratory tract; cilia beat to move the mucus upwards and out of the respiratory tract where it is swallowed or coughed up (2 marks)
- Stomach acid — hydrochloric acid in the stomach kills bacteria and other pathogens in food by creating a highly acidic environment (pH 1–2) that denatures their enzymes (2 marks)
- Tears and saliva — contain lysozyme that breaks down bacterial cell walls; washing action physically removes pathogens (2 marks)
A2. A phagocyte (white blood cell) recognises the pathogen as foreign (1 mark). The phagocyte changes shape and engulfs the pathogen (1 mark). The pathogen is enclosed in a vesicle inside the phagocyte (1 mark). Digestive enzymes break down and destroy the pathogen (1 mark).
A3. Each pathogen has unique antigens on its surface (1 mark). Each antibody has a shape that is complementary to (fits) one specific antigen — this is the lock-and-key mechanism (1 mark). The antibody can only bind to the pathogen with the matching antigen, so a different antibody is needed for each different pathogen (1 mark).
A4. Memory cells are a type of lymphocyte (white blood cell) that remain in the blood after an infection has been fought off (1 mark). If the same pathogen enters the body again, memory cells recognise its antigens and produce the correct antibodies much faster and in greater quantities than during the first infection (1 mark). This means the pathogen is destroyed before it can cause symptoms, providing natural immunity against that disease (1 mark).
A5. Antibodies bind to antigens on the surface of the pathogen itself, neutralising the pathogen or marking it for destruction by phagocytosis (1 mark). Antitoxins bind to and neutralise toxins (poisonous substances) produced by pathogens, preventing the toxins from damaging body cells (1 mark). Both are specific proteins produced by white blood cells, but antibodies target the pathogen while antitoxins target the pathogen's toxins (1 mark).
A6. Goblet cells in the trachea produce mucus, a sticky substance that traps the bacteria and prevents them from moving deeper into the lungs (1 mark). Cilia (tiny hair-like structures on the cells lining the trachea) beat rhythmically (1 mark), moving the mucus with the trapped bacteria upwards and out of the respiratory tract to the throat, where it is swallowed (and destroyed by stomach acid) or coughed out (1 mark).
