B13: The Body's Defences
Primary defences, phagocytosis, antibodies, antitoxins, and memory cells
Key Definitions
Primary defence — The body's first line of defence: physical and chemical barriers that prevent pathogens from entering the body.
Immune system — The body's second line of defence: the white blood cells and immune responses that destroy pathogens that have entered the body.
Phagocytosis — The process by which phagocytes (a type of white blood cell) engulf and digest pathogens.
Antibody — A protein produced by lymphocytes (B-cells) that is specific to a particular antigen on a pathogen; helps destroy the pathogen.
Antitoxin — A protein produced by lymphocytes that neutralises toxins produced by pathogens.
Antigen — A protein on the surface of a pathogen that triggers an immune response; each pathogen has unique antigens.
Immune system — The body's second line of defence: the white blood cells and immune responses that destroy pathogens that have entered the body.
Phagocytosis — The process by which phagocytes (a type of white blood cell) engulf and digest pathogens.
Antibody — A protein produced by lymphocytes (B-cells) that is specific to a particular antigen on a pathogen; helps destroy the pathogen.
Antitoxin — A protein produced by lymphocytes that neutralises toxins produced by pathogens.
Antigen — A protein on the surface of a pathogen that triggers an immune response; each pathogen has unique antigens.
Primary Defences (Non-specific)
Primary defences prevent pathogens from entering the body — they are non-specific (they work against ALL pathogens):
Skin — Acts as a physical barrier; the outer layer is dead, keratinised cells that pathogens cannot penetrate. Sebum (oil) produced by sebaceous glands contains antimicrobial chemicals. If the skin is cut, platelets quickly form a blood clot to seal the wound and prevent pathogen entry.
Mucus — Produced by cells lining the respiratory tract (nose, trachea, bronchi). Mucus is sticky and traps pathogens and dust particles. Ciliated epithelial cells have tiny hair-like cilia that beat rhythmically to move the mucus (with trapped pathogens) up and out of the respiratory tract to be swallowed or coughed out.
Stomach acid — Hydrochloric acid in the stomach creates a very acidic environment (pH ~2) that kills most bacteria and other pathogens that are swallowed with food or mucus.
Tears — Contain lysozyme, an enzyme that breaks down bacterial cell walls, protecting the eye from infection.
Ear wax — Traps pathogens and dust before they can enter the ear canal.
Skin — Acts as a physical barrier; the outer layer is dead, keratinised cells that pathogens cannot penetrate. Sebum (oil) produced by sebaceous glands contains antimicrobial chemicals. If the skin is cut, platelets quickly form a blood clot to seal the wound and prevent pathogen entry.
Mucus — Produced by cells lining the respiratory tract (nose, trachea, bronchi). Mucus is sticky and traps pathogens and dust particles. Ciliated epithelial cells have tiny hair-like cilia that beat rhythmically to move the mucus (with trapped pathogens) up and out of the respiratory tract to be swallowed or coughed out.
Stomach acid — Hydrochloric acid in the stomach creates a very acidic environment (pH ~2) that kills most bacteria and other pathogens that are swallowed with food or mucus.
Tears — Contain lysozyme, an enzyme that breaks down bacterial cell walls, protecting the eye from infection.
Ear wax — Traps pathogens and dust before they can enter the ear canal.
Example 1: How the Skin and Mucus Work as Primary Defences
The skin forms a waterproof, physical barrier that pathogens cannot easily penetrate. If the skin is cut, blood clotting quickly seals the wound. In the respiratory tract, goblet cells produce mucus that traps inhaled bacteria and viruses. Ciliated cells sweep this mucus upward to the throat, where it is swallowed. The stomach acid then destroys the trapped pathogens. This multi-layered defence greatly reduces the number of pathogens reaching the body's internal tissues.
The Immune System (Specific Defences)
If pathogens get past the primary defences, the immune system responds with white blood cells. Unlike primary defences, immune responses are specific — they target particular pathogens.
Phagocytosis
Phagocytes are white blood cells that carry out phagocytosis:
1. The phagocyte recognises the pathogen as foreign (non-self) because of its antigens.
2. The phagocyte changes shape and engulfs the pathogen, enclosing it in a vesicle.
3. Enzymes inside the vesicle digest and destroy the pathogen.
4. The digested products are absorbed by the phagocyte.
Phagocytes are non-specific — they can engulf any type of pathogen. They are the first white blood cells to respond to an infection.
1. The phagocyte recognises the pathogen as foreign (non-self) because of its antigens.
2. The phagocyte changes shape and engulfs the pathogen, enclosing it in a vesicle.
3. Enzymes inside the vesicle digest and destroy the pathogen.
4. The digested products are absorbed by the phagocyte.
Phagocytes are non-specific — they can engulf any type of pathogen. They are the first white blood cells to respond to an infection.
Example 2: Phagocytosis Step by Step
When bacteria enter the body through a wound, phagocytes are attracted to the area by chemical signals. A phagocyte recognises the bacterium as foreign due to its surface antigens. The phagocyte extends its cell membrane around the bacterium, engulfing it completely. The bacterium is enclosed in a phagocytic vesicle (phagosome). Enzymes are released into the vesicle and break down the bacterium. The phagocyte absorbs the digested products and may display the bacterial antigens on its surface to alert other immune cells.
Antibody Production
Lymphocytes (B-lymphocytes / B-cells) are white blood cells that produce antibodies:
1. Each lymphocyte produces a specific antibody that matches only one type of antigen (like a lock and key).
2. When a lymphocyte encounters its matching antigen on a pathogen, it is activated and divides rapidly (clonal selection and expansion).
3. The activated lymphocytes produce large quantities of the specific antibody.
4. Antibodies bind to the antigens on the pathogen, causing:
— Agglutination — Pathogens clump together, making them easier for phagocytes to engulf.
— Neutralisation — Antibodies block the pathogen's ability to infect cells.
— Complement activation — Triggers destruction of the pathogen's cell membrane.
5. Each antibody is specific to one antigen — it will not bind to other pathogens.
1. Each lymphocyte produces a specific antibody that matches only one type of antigen (like a lock and key).
2. When a lymphocyte encounters its matching antigen on a pathogen, it is activated and divides rapidly (clonal selection and expansion).
3. The activated lymphocytes produce large quantities of the specific antibody.
4. Antibodies bind to the antigens on the pathogen, causing:
— Agglutination — Pathogens clump together, making them easier for phagocytes to engulf.
— Neutralisation — Antibodies block the pathogen's ability to infect cells.
— Complement activation — Triggers destruction of the pathogen's cell membrane.
5. Each antibody is specific to one antigen — it will not bind to other pathogens.
Example 3: Lock and Key Model of Antibody Action
Each antibody has a binding site with a unique shape that is complementary to only one specific antigen on a pathogen's surface — like a lock and key. If the antibody does not match the antigen, it cannot bind and has no effect. This is why the immune response is specific: the body produces different antibodies for different pathogens. For example, the antibody that binds to the measles virus antigen will not bind to the Salmonella bacterium antigen.
Antitoxin Production
Some bacteria produce toxins that damage body tissues. Lymphocytes produce antitoxins — specific proteins that bind to and neutralise these toxins, preventing them from causing harm. Antitoxins are a type of antibody that targets toxins rather than the pathogen itself.
Memory Cells
After an infection, some of the activated lymphocytes become memory cells:
— Memory cells remain in the blood for years (sometimes for life).
— If the same pathogen enters the body again, memory cells recognise it immediately and produce antibodies much faster and in greater quantities than the first time.
— The pathogen is usually destroyed before it can cause symptoms — this is natural immunity.
— This is why you usually only get certain diseases once (e.g. measles).
Primary response — The first immune response to a new pathogen; slow and produces few antibodies.
Secondary response — The response when the same pathogen is encountered again; rapid and produces large quantities of antibodies.
— Memory cells remain in the blood for years (sometimes for life).
— If the same pathogen enters the body again, memory cells recognise it immediately and produce antibodies much faster and in greater quantities than the first time.
— The pathogen is usually destroyed before it can cause symptoms — this is natural immunity.
— This is why you usually only get certain diseases once (e.g. measles).
Primary response — The first immune response to a new pathogen; slow and produces few antibodies.
Secondary response — The response when the same pathogen is encountered again; rapid and produces large quantities of antibodies.
Example 4: Primary vs Secondary Immune Response
When a person is infected with measles for the first time, the primary immune response takes 7–14 days to produce enough antibodies to clear the infection — during which time the person experiences symptoms. Memory cells are produced. If the same person is exposed to measles again, the secondary response is triggered within hours, producing antibodies so quickly that the virus is destroyed before symptoms develop. The person is immune.
Exam tip: When describing the immune response, clearly distinguish between phagocytes (non-specific, engulf pathogens) and lymphocytes (specific, produce antibodies and antitoxins). Always use the word "specific" when describing antibody action — antibodies are complementary in shape to a particular antigen.
Practice Questions
1. Foundation Name three primary defences of the body and describe how each prevents pathogens from entering.
1) Skin — acts as a physical barrier preventing pathogens from entering; blood clots seal any cuts. 2) Mucus — sticky substance in the respiratory tract that traps pathogens; cilia sweep mucus out of the airways. 3) Stomach acid — hydrochloric acid at pH ~2 kills most pathogens that are swallowed. Also acceptable: tears (contain lysozyme enzyme), ear wax.
2. Foundation Describe the process of phagocytosis.
A phagocyte (white blood cell) recognises a pathogen as foreign due to its antigens. The phagocyte changes shape, engulfs the pathogen, and encloses it in a vesicle. Enzymes inside the vesicle digest and destroy the pathogen. The digested products are absorbed by the phagocyte.
3. Higher Explain how antibodies are specific to particular pathogens.
Each antibody has a binding site with a unique three-dimensional shape that is complementary to only one specific antigen on a pathogen's surface (lock and key model). Only the correct antibody can bind to a particular antigen. If the antibody's binding site does not match the antigen, it cannot attach and has no effect. This means each antibody only works against the pathogen whose antigens it matches.
4. Higher Explain the role of memory cells in immunity.
After an infection, some activated lymphocytes become memory cells and remain in the blood for years. If the same pathogen enters the body again, memory cells recognise it immediately and produce antibodies much faster and in greater quantities than during the first infection. The pathogen is destroyed before it can cause symptoms, so the person is immune to that disease. This is why most people only get diseases like measles once.
5. Higher Compare the roles of phagocytes and lymphocytes in the immune response.
Phagocytes are non-specific — they can engulf and digest any pathogen by phagocytosis. They are the first white blood cells to respond. Lymphocytes are specific — each produces one type of antibody that matches only one pathogen's antigens. Lymphocytes also produce antitoxins to neutralise bacterial toxins and form memory cells for long-term immunity. Phagocytes destroy pathogens directly; lymphocytes destroy them indirectly using antibodies and antitoxins.
🔢 Maths Skills
Mathematical Skills
Minimal quantitative skills for this topic. You may need to interpret simple counts or comparisons of white blood cell types in data tables.
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Antibodies and antibiotics are the same thing Correct: They are completely different — antibodies are proteins produced by white blood cells that target specific pathogens; antibiotics are medicines that kill bacteria
2. Wrong: Phagocytosis is the only way white blood cells defend the body Correct: White blood cells also produce antibodies and antitoxins — phagocytosis is just one of three defence mechanisms
✍️ 6-Mark Question
Extended Answer
6 marks: Explain the three ways white blood cells defend against pathogens.
White blood cells defend the body in three ways. First, phagocytes carry out phagocytosis — they engulf and digest pathogens, destroying them directly. Second, lymphocytes produce specific antibodies that bind to antigens on the pathogen, causing agglutination and neutralisation. Third, lymphocytes produce antitoxins that neutralise toxins produced by bacteria, preventing them from damaging body tissues. Memory cells are also formed, providing long-term immunity.
Mark scheme: 1 mark for phagocytosis with explanation; 1 mark for antibody production with specificity; 1 mark for antitoxin production; 1 mark for detail of each mechanism (engulf/digest, agglutination/neutralisation, neutralise toxins); 1 mark for mentioning memory cells; 1 mark for QWC (clear, organised response with correct terminology)
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A study measures antibody levels in blood samples taken from patients at different times after infection. The data shows: Day 0 = 0 arbitrary units; Day 7 = 15; Day 14 = 80; Day 21 = 120 (peak); Day 28 = 90. A second exposure on Day 60 produces: Day 61 = 60; Day 63 = 200; Day 67 = 350.
1. Describe the difference between the primary and secondary immune response shown by the data. 2. Explain why the secondary response is faster and stronger. 3. Evaluate the claim that "vaccination would produce the same pattern" — is this justified by the data?
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