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B10: Body Defences
FoundationHigher
The body's defence systems against pathogens — physical and chemical barriers, and the role of the immune system's white blood cells.
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.
Primary Defences (Physical and Chemical Barriers)
Primary defences are the body's first line of defence. They are non-specific — they work against all pathogens, not just one type. Their job is to prevent pathogens from entering the body.
Skin
The skin is a physical barrier that covers the body and prevents pathogens from entering tissues beneath
If the skin is cut, blood clots quickly seal the wound to prevent entry of pathogens
The skin also produces antimicrobial secretions (sebum) that kill bacteria on the surface
Sweat contains an enzyme called lysozyme that breaks down bacterial cell walls
Mucus Membranes
The respiratory tract (nose, trachea, bronchi) is lined with mucus-producing cells
Mucus is a sticky substance that traps pathogens and dust particles in the air you breathe in
Cilia (tiny hair-like structures on the cells lining the trachea) beat rhythmically to move the mucus (with trapped pathogens) upwards and out of the respiratory tract, where it is swallowed or coughed out
Stomach Acid
The stomach produces hydrochloric acid (HCl) with a pH of approximately 1–2
This highly acidic environment kills most bacteria and other pathogens that enter the stomach in food or in swallowed mucus
The acid denatures enzymes in the pathogens, preventing them from functioning
Tears and Saliva
Tears and saliva contain lysozyme, an enzyme that breaks down the cell walls of bacteria
Tears wash the surface of the eye, physically removing pathogens
Saliva washes the mouth and continuously flushes bacteria down to the stomach where acid destroys them
Primary Defence
Type
How It Works
Skin
Physical + Chemical
Barrier + antimicrobial secretions + lysozyme in sweat
Mucus membranes + cilia
Physical
Mucus traps pathogens; cilia move mucus out of respiratory tract
Stomach acid
Chemical
Low pH kills bacteria and denatures pathogen enzymes
Tears and saliva
Chemical + Physical
Lysozyme breaks down bacterial cell walls; washing action removes pathogens
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)
Practice Questions
Q1. Name three primary defences of the human body and describe how each one prevents pathogens from entering. Foundation [6 marks]
Q2. Describe the process of phagocytosis. FoundationHigher [4 marks]
Q3. Explain how antibodies are specific to one type of pathogen. Use the term 'lock-and-key' in your answer. FoundationHigher [3 marks]
Q4. What are memory cells and why are they important in the immune response? Higher [3 marks]
Q5. Explain the difference between antibodies and antitoxins. FoundationHigher [3 marks]
Q6. A person breathes in bacteria that are trapped by mucus in their trachea. Describe how the mucus and cilia work together to remove these bacteria from the respiratory tract. Foundation [3 marks]
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).
Exam Tips
Primary defences are NON-SPECIFIC. They work against all pathogens. The immune system (white blood cells) includes both non-specific (phagocytosis) and specific (antibodies, antitoxins) responses.
Don't mix up antigens and antibodies. Antigens are on the surface of the pathogen. Antibodies are produced by white blood cells to bind to antigens. Antigens trigger the immune response; antibodies carry it out.
Don't mix up antibodies and antitoxins. Antibodies target the pathogen itself. Antitoxins target the toxins produced by the pathogen.
"Explain" means give reasons. "The skin prevents infection" is a description. "The skin prevents infection because it acts as a physical barrier that pathogens cannot pass through, and it produces antimicrobial secretions that kill bacteria" is an explanation.
Use the term "complementary shape" when describing how antibodies bind to antigens. The word "complementary" scores marks in exams — "same shape" or "matching shape" is less precise.
Memory cells are the basis of immunity (and vaccination in B11). Remember: they allow a faster, stronger secondary response so you don't get ill a second time from the same pathogen.
Phagocytosis is non-specific — phagocytes can engulf any pathogen. Antibody production is specific — each antibody fits only one antigen type.
🔢 Maths Skills
Mathematical Skills
Mathematical skills in this topic are minimal. You may need to interpret data on white blood cell counts or antibody levels over time from line graphs.
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Antibodies and antibiotics are the same thingCorrect: Antibodies are proteins produced by white blood cells that target specific pathogens; antibiotics are drugs (medicines) that kill bacteria — they are completely different substances
2. Wrong: Phagocytosis is the body's only defence against pathogensCorrect: The body also defends using antibodies (which target specific pathogens), antitoxins (which neutralise toxins), and physical/chemical barriers (skin, mucus, stomach acid)
✍️ 6-Mark Question
Extended Answer
6 marks: Explain three ways white blood cells defend the body against pathogens.
Firstly, phagocytes carry out phagocytosis — they recognise pathogens as foreign, change shape to surround and engulf them, and use digestive enzymes to destroy them. This is a non-specific defence that works against any pathogen. Secondly, lymphocytes produce antibodies — proteins with a shape complementary to the antigens on a specific pathogen (lock-and-key mechanism). Antibodies bind to the pathogen, clumping them together for easier phagocytosis and neutralising them. After the infection, memory cells remain so the body can respond faster next time. Thirdly, white blood cells produce antitoxins — these are specific proteins that bind to and neutralise toxins produced by bacteria, preventing the toxins from damaging body cells. Unlike antibodies which target the pathogen, antitoxins target the harmful chemicals the pathogen releases.
Mark scheme: 1 mark for phagocytosis description; 1 mark for phagocytosis detail (engulf/digest); 1 mark for antibodies description; 1 mark for antibody specificity (lock-and-key); 1 mark for antitoxins description; 1 mark for distinction between antibodies and antitoxins
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A graph shows antibody levels in the blood after a person is infected with a pathogen for the first time and then again 6 months later.
(a) Describe the difference in antibody production between the first and second exposure.
(b) Explain why the second response is different. Why does this person not get ill the second time?
Answers: (a) On first exposure, antibodies are produced slowly and reach a lower peak. On second exposure, antibodies are produced much faster and reach a much higher concentration. (b) Memory cells from the first infection remain in the blood. On second exposure, they recognise the pathogen's antigens immediately and produce the correct antibodies rapidly and in large quantities, destroying the pathogen before it can multiply enough to cause symptoms.
