Ch 1 · Diseases and Immunity · IGCSE Biology 0610
Topic 1 of 1 in Diseases and Immunity — IGCSE Biology 0610 — 58 lessons.
Introduction
The body meets disease-causing microorganisms every day. Most never get in; those that do are destroyed by white blood cells. This lesson covers pathogens and transmissible disease, the barriers that keep pathogens out, public health measures and, at Extended level, how the immune system recognises specific pathogens, how vaccination works and why memory cells make immunity last.
Key ideas — Pathogens and transmissible diseases
A pathogen is a disease-causing organism: bacteria (cholera), viruses (influenza), fungi (athlete's foot) and protoctists (malaria).
🧑🏫 Teacher's explanation
Think of your body as a fortified castle. The first line of defence is the wall (your skin) and the moat (mucus). If a pathogen—a disease-causing organism like a bacterium or virus—gets past these barriers, the castle guards (white blood cells) attack. Some guards eat the invaders (phagocytosis), while others throw spears (antibodies) that fit only specific invaders.
📖 Definitions
🔍 Worked example — Easy
Define a pathogen and give one example of a bacterial pathogen and one of a viral pathogen.
🔍 Worked example — Standard
Explain how the body defends itself against pathogens using mechanical and chemical barriers.
🔍 Worked example — Exam-style
Explain how vaccination provides immunity and why herd immunity is important.
⚠️ Common errors
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🎯 Exam question
(a) Describe how cholera is transmitted. [2] (b) Explain how the body's immune system responds to the *Vibrio cholerae* bacterium. [3] (c) Suggest two ways to prevent the spread of cholera in a community. [2] [7 marks]
Key ideas — The body's defences
🧑🏫 Teacher's explanation
Think of your body like a high-security bank. The skin is the outer wall and the hairs in your nose are the air filters; they are the first line of defence, physically stopping most intruders (pathogens) from entering. If a pathogen slips through a cut or is swallowed, the chemical barriers take over. Stomach acid is like a vat of bleach that dissolves anything that shouldn't be there, while mucus acts like sticky glue, trapping microbes so they can be swept away.
📖 Definitions
🔍 Worked example — Easy
Describe the role of the skin and stomach acid in the body's defence against pathogens.
🔍 Worked example — Standard
Explain how phagocytosis and antibody production differ in their mechanism of destroying pathogens.
🔍 Worked example — Exam-style
A town has a high rate of cholera. Suggest two measures involving water and hygiene to control the spread of this transmissible disease, and explain how they work.
⚠️ Common errors
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🎯 Exam question
Key ideas — Controlling the spread of disease
🧑🏫 Teacher's explanation
Think of your body as a fortress. The walls are your mechanical barriers like skin and nose hairs, which stop invaders from entering. The moat is your chemical barriers, such as stomach acid and mucus, which trap or dissolve anything that slips through. Inside the castle, the soldiers are your white blood cells, which hunt down and destroy any pathogen that manages to breach the outer defenses.
📖 Definitions
🔍 Worked example — Easy
Explain how the body’s mechanical and chemical barriers help prevent the entry of pathogens.
🔍 Worked example — Standard
A town experiences an outbreak of cholera. Suggest two measures involving water and waste management that could control the spread, and explain how each works.
🔍 Worked example — Exam-style
Explain how vaccination provides active immunity and why herd immunity is important for the protection of the community.
⚠️ Common errors
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🎯 Exam question
(a) Explain how the body’s immune system responds to the Vibrio cholerae bacterium once it has entered the body. [3]
Key ideas — Antigens, antibodies and active immunity
🧑🏫 Teacher's explanation
Imagine your immune system is a security team at a high-security building. The 'bad guys' (pathogens) have unique keycards (antigens) on their jackets. The security guards (lymphocytes) don't just stop anyone; they look for a specific keycard shape. If they see a keycard they don't recognize, they create a custom handcuff (antibody) that fits that exact keycard shape. This is active immunity: your body learns to make the handcuff itself.
📖 Definitions
🔍 Worked example — Easy
A student is exposed to the measles virus. Describe the sequence of events that leads to active immunity in this student.
🔍 Worked example — Standard
Compare active immunity gained through vaccination with passive immunity gained through breast milk. State two differences.
🔍 Worked example — Exam-style
A town has a high vaccination rate for polio. Explain why this protects unvaccinated individuals in the town, and why the polio vaccine does not protect against the flu.
⚠️ Common errors
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🎯 Exam question
Key ideas — Memory cells, vaccination and passive immunity
🧑🏫 Teacher's explanation
Think of your immune system like a city's defence force. The skin is the city wall, and white blood cells are the soldiers. When a new enemy (pathogen) attacks, the soldiers fight, but it takes time. Some soldiers become 'memory soldiers' who remember the enemy's face (antigen). If the same enemy returns, these memory soldiers react instantly and in huge numbers, stopping the attack before you even feel sick. This is active immunity.
📖 Definitions
🔍 Worked example — Easy
A student receives a vaccine for measles. Explain why the student is protected against measles for many years, but why the student is NOT protected against the flu.
🔍 Worked example — Standard
A newborn baby is healthy and has antibodies against tetanus in its blood. The baby has never been vaccinated and has never had tetanus. Explain how the baby acquired these antibodies and why this protection will eventually disappear.
🔍 Worked example — Exam-style
In a town, 95% of the population is vaccinated against polio. A person who is not vaccinated moves into the town. Explain why this unvaccinated person is likely to be safe from polio, referring to the concept of herd immunity.
⚠️ Common errors
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🎯 Exam question
(b) Explain how vaccination can lead to herd immunity. (2 marks) (c) A patient is given an injection of antibodies against a venomous snake bite. Explain why this provides immediate but short-term protection. (2 marks) [7 marks]
Worked example
Question. (a) Define a transmissible disease. [2] (b) Explain how sewage treatment reduces the spread of cholera. [2] (c) (Supplement) A child is vaccinated against measles. Explain how the vaccine gives long-term immunity. [4] (d) (Supplement) Explain why the child is still at risk of catching mumps. [2]
🔍 Worked example — Easy
Define a pathogen and give one example of a mechanical barrier that prevents pathogens from entering the body.
🔍 Worked example — Standard
Explain how the body uses phagocytosis to defend against a bacterial infection that has breached the skin.
🔍 Worked example — Exam-style
A student is vaccinated against tetanus. Two years later, they are exposed to the tetanus bacterium. Explain why they do not develop the disease, referring to the role of memory cells.
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Practice
Answers