Topic 10 of IGCSE Biology (Cambridge 0610) is diseases and immunity. You need to define pathogens and name the four types (virus, bacterium, fungus, protoctist) with an example each, describe transmission of disease and how spread can be reduced, state the role of body defences (skin, mucus, stomach acid, white blood cells), and explain immunity (active, passive, vaccination). This revision guide takes you through each of these with clear explanations and exam-style tips for full marks.

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Pathogens: the four types

A pathogen is an organism (or virus) that causes disease. The four types you need to know are: virus (e.g. flu virus, HIV, measles virus); bacterium (e.g. Salmonella, tuberculosis bacteria); fungus (e.g. athlete’s foot fungus, yeast infections); protoctist (e.g. Plasmodium, which causes malaria; Amoeba). Viruses are not usually classed as “living” because they do not carry out all life processes on their own; they need to invade a host cell to reproduce. You may be asked to define pathogen and to name the four types with one example each.

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Transmission of disease

Diseases can be transmitted (spread) in several ways: direct contact (e.g. skin contact, sexual contact); water (e.g. cholera in contaminated water); air (e.g. droplets from coughs and sneezes — flu, tuberculosis); vectors (e.g. mosquitoes carrying malaria, ticks carrying Lyme disease). To reduce spread, we use: hygiene (hand washing, clean water, safe disposal of waste); vaccination (so fewer people catch and pass on the disease); isolating infected people; vector control (e.g. mosquito nets, insecticides). You may be asked to describe how one named disease is transmitted and to suggest one way to reduce its spread.

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Body defences: skin, mucus, stomach acid, white blood cells

The body has several non-specific defences that act against many pathogens. The skin is a barrier that prevents pathogens from entering; cuts and grazes can allow entry. Mucus (in the nose, trachea, and bronchi) traps pathogens; cilia (tiny hairs) waft the mucus up to the throat so it can be swallowed or coughed out. Stomach acid (hydrochloric acid) kills many pathogens that are swallowed. White blood cells defend in two main ways: phagocytosis (engulfing and digesting pathogens) and antibodies (proteins that bind to specific antigens on pathogens and help destroy them or mark them for destruction). You may be asked to state the role of skin, mucus, stomach acid, and white blood cells in defence.

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Antibodies and antigens

An antigen is a molecule (often on the surface of a pathogen) that triggers an immune response. An antibody is a protein produced by white blood cells that binds to a specific antigen. Each antibody fits a specific antigen (lock and key). When antibodies bind to antigens, they may neutralise the pathogen, mark it for destruction by other cells, or clump pathogens together. Do not confuse antibody (protein that binds to antigen) with antigen (molecule that triggers the response). Vaccination works by introducing antigens (or weakened/dead pathogens) so that the body produces antibodies and memory cells; on re-infection, the response is faster and stronger.

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Active and passive immunity; vaccination

Active immunity is when the body produces its own antibodies (e.g. after infection or after vaccination). It is usually long-lasting because memory cells remain and can produce antibodies quickly if the pathogen is encountered again. Passive immunity is when antibodies are given to a person (e.g. from a mother to her baby across the placenta or in breast milk; or in an injection of antibodies). It is short-lived because the body does not make its own antibodies and they are gradually broken down.

Vaccination is a way of giving active immunity: a vaccine contains weakened or dead pathogens, or antigens from the pathogen. This stimulates the body to produce antibodies and memory cells without causing the full disease. If the person is later infected by the real pathogen, the memory cells produce antibodies quickly, so the disease is prevented or much milder. Booster doses may be needed because immunity can fade over time. You may be asked to explain how vaccination leads to immunity (stimulates antibody production; memory cells; rapid response on re-infection) and why boosters are sometimes used. Do not say vaccination “kills” the pathogen; the vaccine stimulates the immune response.

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Exam tips and command words

• State: Name the pathogen or the type of transmission.
• Describe: How a disease is spread or how the body defends against infection.
• Explain: Why vaccination protects (antibodies; memory cells; rapid response on re-infection).
• Compare: Active vs passive immunity (who makes antibodies; how long it lasts).

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Common mistakes to avoid

• Saying viruses are “living” (they do not carry out all life processes; they need a host).
• Confusing antibody (protein that binds to antigen) with antigen (molecule that triggers immune response).
• Saying vaccination “kills” the pathogen (vaccine stimulates the immune system; body makes antibodies).
• Not stating that active immunity is long-lasting (memory cells) and passive is short-lived.

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Revision checklist

• Define pathogen and name the four types with one example each.
• Describe how one named disease is transmitted and how spread can be reduced.
• State the role of skin, mucus, stomach acid, and white blood cells in defence.
• Explain how vaccination leads to immunity and why booster doses may be needed.

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Next steps

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