Immune system and immunity

What you'll learn

The immune system protects the body from pathogens through physical barriers and coordinated cellular responses. This topic covers how the body recognises and destroys disease-causing microorganisms, the role of white blood cells, and how immunity develops through natural infection or vaccination. Understanding these processes is essential for explaining disease prevention and treatment strategies.

Key terms and definitions

Pathogen — a disease-causing microorganism such as bacteria, viruses, fungi or protoctists

Antigen — a protein on the surface of a cell that triggers an immune response; pathogens have antigens that the immune system recognises as foreign

Antibody — a Y-shaped protein produced by lymphocytes that binds to specific antigens on pathogens, leading to their destruction

Phagocytosis — the process by which phagocytes engulf and digest pathogens

Active immunity — immunity developed when the immune system produces its own antibodies after exposure to antigens, either through infection or vaccination

Passive immunity — short-term immunity gained when antibodies are transferred from another individual, such as from mother to baby

Vaccination — the introduction of weakened or dead pathogens (or their antigens) into the body to stimulate antibody production without causing disease

Memory cells — specialised lymphocytes that remain in the blood after an infection, enabling rapid antibody production if the same pathogen is encountered again

Core concepts

Physical and chemical barriers to infection

The body's first line of defence prevents pathogens from entering in the first place. These non-specific defences work against all types of pathogens.

Skin acts as a physical barrier:

• The outer layer consists of dead cells that pathogens cannot penetrate
• Sebaceous glands secrete oils that kill bacteria
• If the skin is cut, blood clotting seals the wound rapidly

Mucous membranes line the respiratory, digestive and reproductive tracts:

• Mucus traps pathogens and particles
• Cilia (tiny hair-like structures) in the trachea and bronchi sweep mucus upward to be swallowed
• Stomach acid destroys most pathogens that are swallowed

Chemical defences include:

• Lysozyme in tears, which breaks down bacterial cell walls
• Hydrochloric acid in the stomach (pH 1.5-2.0), which denatures pathogen enzymes
• Antibacterial secretions in the vagina

White blood cells and the immune response

If pathogens breach the physical barriers, the immune system provides the second line of defence. White blood cells (leucocytes) coordinate this response. The two main types relevant to IGCSE are phagocytes and lymphocytes.

Phagocytes carry out phagocytosis:

1. The phagocyte detects chemicals released by pathogens and moves toward them
2. The phagocyte's cell membrane extends around the pathogen, engulfing it
3. The pathogen is enclosed in a vacuole inside the phagocyte
4. Enzymes are released into the vacuole
5. The enzymes digest and destroy the pathogen

This process is non-specific — phagocytes attack any pathogen they encounter.

Lymphocytes produce antibodies in a specific immune response:

• Each lymphocyte produces one type of antibody with a specific shape
• The antibody shape is complementary to a particular antigen
• When a lymphocyte encounters its matching antigen, it is activated
• The lymphocyte divides rapidly to produce many identical cells (clones)
• These cells produce large quantities of the specific antibody
• Antibodies bind to antigens on the pathogen's surface

Antibodies destroy pathogens by:

• Causing pathogens to clump together (agglutination), making them easier for phagocytes to engulf
• Coating pathogens so phagocytes recognise them more easily
• Neutralising toxins produced by bacteria
• Damaging or rupturing pathogen cell membranes

Active immunity

Active immunity provides long-lasting protection because the immune system produces its own antibodies and memory cells.

Natural active immunity develops after infection:

1. Pathogens enter the body and begin to multiply
2. Symptoms of disease may appear during this time
3. Lymphocytes produce specific antibodies (this takes several days)
4. Antibodies destroy the pathogens and symptoms disappear
5. Some lymphocytes become memory cells that remain in the blood
6. If the same pathogen enters again, memory cells rapidly produce large quantities of antibodies
7. The pathogen is destroyed before symptoms develop — the person is immune

Artificial active immunity develops after vaccination:

• Vaccines contain weakened or dead pathogens, or isolated pathogen antigens
• These cannot cause disease but carry the antigens needed to trigger an immune response
• Lymphocytes produce antibodies against the antigens
• Memory cells form, providing immunity without suffering the disease
• Booster vaccinations may be needed to maintain high antibody levels

The primary response (first exposure) is slow — antibody levels take 7-14 days to peak. The secondary response (second exposure) is rapid — antibody levels peak within 2-3 days and reach higher concentrations.

Passive immunity

Passive immunity provides immediate but temporary protection because antibodies come from another source.

Natural passive immunity:

• Antibodies pass from mother to baby across the placenta during pregnancy
• Antibodies also pass through breast milk (colostrum is especially rich in antibodies)
• This protects the baby during the first months of life while its immune system develops
• The antibodies gradually break down and are not replaced

Artificial passive immunity:

• Ready-made antibodies are injected into a person
• Used when immediate protection is needed (e.g., after exposure to rabies or tetanus)
• Also used for immunocompromised patients who cannot produce their own antibodies
• Protection lasts only a few weeks as the antibodies are broken down

Vaccination programmes and herd immunity

Vaccination programmes protect individuals and communities from infectious diseases.

Individual benefits:

• Prevents specific diseases (e.g., measles, polio, tetanus, diphtheria)
• Avoids complications from disease (e.g., brain damage from measles, paralysis from polio)
• More cost-effective than treating disease

Community benefits — herd immunity:

• When a high percentage of the population is vaccinated (typically 85-95%), disease transmission is reduced
• Protects vulnerable individuals who cannot be vaccinated (babies, immunocompromised people)
• Can lead to disease eradication (smallpox was eradicated globally in 1980)

Factors affecting vaccination programme success:

• Vaccine availability and cost
• Public education about benefits and safety
• Cultural and religious beliefs
• Access to healthcare services
• Pathogen mutation rates (e.g., influenza mutates rapidly, requiring new vaccines annually)

Common vaccines in UK/Caribbean programmes include:

• MMR (measles, mumps, rubella)
• DTP (diphtheria, tetanus, pertussis)
• Polio
• HPV (human papillomavirus)
• Influenza (for at-risk groups)

Antibiotics and antibiotic resistance

Antibiotics are drugs that kill bacteria or prevent their growth. They are produced naturally by some microorganisms (e.g., penicillin from Penicillium mould).

Important points about antibiotics:

• They work by damaging bacterial cell walls or interfering with bacterial protein synthesis
• They are ineffective against viruses because viruses reproduce inside host cells and lack the structures antibiotics target
• Different antibiotics work against different bacteria (broad-spectrum vs narrow-spectrum)

Antibiotic resistance occurs when bacteria evolve to survive antibiotic treatment:

Development of resistance:

1. Random mutations in bacterial DNA create genetic variation
2. Some mutations make bacteria resistant to specific antibiotics
3. When antibiotics are used, non-resistant bacteria die
4. Resistant bacteria survive and reproduce rapidly
5. Resistance genes are passed to offspring (and sometimes to other bacteria)
6. Resistant strains become dominant (natural selection)

Examples include MRSA (methicillin-resistant Staphylococcus aureus), which causes difficult-to-treat infections in hospitals.

Reducing antibiotic resistance:

• Complete the full course of antibiotics (prevents partially resistant bacteria surviving)
• Only use antibiotics for bacterial infections, not viral infections
• Avoid overuse in agriculture (e.g., routine use in animal feed)
• Develop new antibiotics
• Improve hygiene in hospitals to prevent spread of resistant bacteria
• Use narrow-spectrum antibiotics when possible

Worked examples

Example 1: Describe how phagocytes destroy pathogens that enter the body. [4 marks]

Mark scheme answer:

• Phagocyte detects chemicals from pathogens / moves toward pathogen [1]
• Cell membrane extends around the pathogen / engulfs the pathogen [1]
• Pathogen enclosed in a vacuole/vesicle [1]
• Enzymes digest/break down the pathogen [1]

Key points: Use precise terminology (engulf, vacuole, digest). Describe the process in sequence. Four separate points needed for four marks.

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Example 2: Explain why a person who has recovered from measles is unlikely to catch measles again, but may still catch chickenpox. [4 marks]

Mark scheme answer:

• After measles infection, the person has produced specific antibodies [1]
• Memory cells remain in the blood [1]
• If measles pathogen enters again, memory cells rapidly produce antibodies / secondary response is faster [1]
• Measles and chickenpox have different antigens / antibodies are specific to one disease only [1]

Key points: Explain the role of memory cells. Address both diseases. Link antibody specificity to antigens.

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Example 3: A baby receives antibodies from its mother through the placenta and breast milk. Explain why this type of immunity is only temporary. [3 marks]

Mark scheme answer:

• This is passive immunity / antibodies come from another person [1]
• The baby has not produced its own antibodies / no memory cells formed [1]
• Antibodies are (proteins that are) gradually broken down / not replaced [1]

Key points: Identify the type of immunity. Explain why it doesn't last — focus on the lack of memory cells and antibody breakdown.

Common mistakes and how to avoid them

• Confusing antibodies with antibiotics. Antibodies are proteins made by lymphocytes that bind to specific antigens. Antibiotics are drugs that kill bacteria. They are completely different things despite similar names.

• Saying "white blood cells produce antigens." Pathogens carry antigens on their surface. White blood cells (specifically lymphocytes) produce antibodies in response to antigens. Make sure you have the direction correct.

• Claiming vaccines contain antibodies. Vaccines contain antigens (from weakened/dead pathogens) that stimulate the body to produce its own antibodies. This is active immunity. Ready-made antibodies are given for passive immunity only.

• Writing that antibiotics kill viruses. Antibiotics only work on bacteria. Be explicit about this distinction in exam answers. If asked why antibiotics don't work on viruses, explain that viruses lack the structures (cell walls, protein synthesis machinery) that antibiotics target.

• Forgetting specificity in immunity answers. Each antibody has a specific shape complementary to one antigen. Each lymphocyte produces one type of antibody. Memory cells are specific to one pathogen. Include "specific" when describing antibody-antigen interactions.

• Describing phagocytosis as "eating." Use scientific terminology: engulfing, enclosing in a vacuole, digesting with enzymes. Avoid informal language in extended response questions.

Exam technique for "Immune system and immunity"

• "Describe" questions require a sequential account of a process. For phagocytosis or antibody production, work through each step logically. Use linking words (then, next, after this) to show the sequence clearly.

• "Explain" questions need reasons and mechanisms. Don't just state what happens — explain why or how. Link cause to effect (e.g., "memory cells enable rapid antibody production, therefore pathogens are destroyed before symptoms develop").

• Mark allocation guides your detail. A 2-mark question needs two distinct points. A 4-mark question typically requires four points or two points with development. If you've written one sentence for 4 marks, you haven't written enough.

• Key command words: "Compare" requires similarities AND differences between active/passive immunity or primary/secondary response. Make direct comparisons rather than describing each separately. "Suggest" means apply your knowledge to unfamiliar contexts — use biological principles to reason through new scenarios.

Quick revision summary

The body defends against pathogens using physical barriers (skin, mucus, stomach acid) and the immune system. Phagocytes engulf pathogens non-specifically. Lymphocytes produce specific antibodies complementary to pathogen antigens. Active immunity (from infection or vaccination) is long-lasting because memory cells enable rapid secondary responses. Passive immunity (from antibodies transferred from another individual) is temporary. Vaccination programmes create herd immunity. Antibiotics kill bacteria but not viruses. Antibiotic resistance evolves through natural selection when antibiotics are overused.