Diseases and Immunity

This topic follows one storyline: a pathogen gets in, the body tries to keep it out, and if it succeeds in entering, the immune system responds — and remembers.

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1. Pathogens and transmission

A pathogen is a disease-causing organism.

A transmissible disease is one in which the pathogen can be passed from one host to another.

Pathogens include bacteria (cholera), viruses (HIV, influenza), fungi and protoctists (malaria).

Routes of transmission

Route	How	Examples
Direct contact	Through blood or other body fluids	HIV
Indirect	Via contaminated food or water, airborne droplets, or animals (vectors)	Cholera, influenza, malaria

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2. The body’s defences

The body has barriers that stop pathogens getting in at all:

Defence	How it works
Skin	A physical barrier covering the body
Hairs in the nose	Trap particles and pathogens in inhaled air
Mucus	Traps pathogens in the airways; cilia sweep it away
Stomach acid	Kills pathogens in swallowed food and mucus
Clotting	Seals wounds, preventing entry

Controlling the spread of disease

• Personal hygiene — hand washing, especially before food and after the toilet
• Clean water supplies and proper sewage treatment
• Waste disposal
• Food hygiene — thorough cooking and correct storage
• Vaccination programmes

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3. Cholera

Cholera is caused by the bacterium Vibrio cholerae, transmitted in contaminated water.

The sequence — worth learning as a chain, because each link is a mark:

1. The bacterium is swallowed in contaminated water or food
2. It reaches the small intestine and produces a toxin
3. The toxin causes the intestine cells to secrete chloride ions into the lumen
4. This lowers the water potential of the intestine contents
5. Water moves out of the cells and blood into the lumen by osmosis
6. This causes severe watery diarrhoea, and so dehydration and loss of ions

It is chloride ions, not chlorine. Chlorine is the element; chloride is the ion. Writing “chlorine ions” is marked wrong.

Do not stop before “diarrhoea”. The final consequence is the easiest mark in the sequence and the one most often left off, because students describe the mechanism and forget to state the outcome.

Treatment is with oral rehydration therapy — replacing lost water and ions.

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4. The immune response

Term	Definition
Antigen	A molecule (often on the surface of a pathogen) that the body recognises as foreign
Antibody	A protein made by lymphocytes that binds to a specific antigen

How it works:

1. A pathogen enters, carrying antigens on its surface
2. Lymphocytes recognise the antigens as foreign
3. They produce antibodies with a shape complementary to that specific antigen
4. Antibodies bind to the antigens, causing pathogens to clump together (agglutinate) and marking them for destruction
5. Phagocytes then engulf and digest the pathogens
6. Some lymphocytes become memory cells, which remain in the body

Antibodies are specific. Each has a shape complementary to one antigen only — the same complementary-shape idea as enzymes and their substrates.

Lymphocytes produce antibodies; phagocytes engulf. Confusing these two is the most common error in the topic. The hook is phagocyte → phagocytosis → engulfing.

The role of memory cells explains why you usually catch a disease only once. On a second exposure, memory cells recognise the antigen and produce antibodies faster and in greater quantity, destroying the pathogen before symptoms develop.

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5. Active and passive immunity

	Active immunity	Passive immunity
How it arises	The body makes its own antibodies after antigens enter	Antibodies are given from another individual
Memory cells?	Yes	No
Speed of protection	Slower to develop	Immediate
Duration	Long-lasting	Short-term
Examples	Catching a disease; vaccination	Antibodies crossing the placenta; antibodies in breast milk

Passive immunity gives no memory cells, which is why it is short-lived. This is the point of comparison examiners return to most often.

Breast milk transfers antibodies, not white blood cells. A mother passes on the antibodies she has made; she does not transfer her lymphocytes.

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6. Vaccination

The process — a four-step answer:

1. A vaccine containing weakened or inactive pathogens, or their antigens, is put into the body
2. The antigens stimulate lymphocytes to produce antibodies
3. Memory cells are produced
4. If the real pathogen enters later, memory cells produce antibodies faster and in greater quantity, destroying it before symptoms develop

A vaccine does not contain antibodies. It contains the weakened or inactive pathogen (or its antigens). Getting this backwards makes the whole explanation collapse, because the point is that the body makes its own antibodies — which is what produces the memory cells.

Herd immunity: if enough people in a population are vaccinated, the pathogen cannot spread easily, so even unvaccinated individuals are protected — because there are too few susceptible people for transmission to continue.

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7. Mistakes that cost marks

Saying phagocytes produce antibodies. Lymphocytes do.

Saying a vaccine contains antibodies. It contains a weakened or inactive pathogen.

Writing “chlorine ions” in the cholera answer.

Leaving “diarrhoea” off the end of the cholera chain.

Saying passive immunity produces memory cells. It does not.

Saying antibodies “kill” pathogens directly. They bind to antigens, causing clumping and marking them for phagocytes.

Confusing antigen with antibody. The antigen is on the pathogen; the antibody is made by the body.

Saying you become immune “because your body gets used to it.” The mechanism is memory cells.

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Frequently asked questions

What is a pathogen? A disease-causing organism.

What is the difference between an antigen and an antibody? An antigen is a foreign molecule, usually on a pathogen’s surface. An antibody is a protein made by lymphocytes that binds to a specific antigen.

Which cells make antibodies? Lymphocytes. Phagocytes engulf pathogens.

How does vaccination work? A weakened or inactive pathogen stimulates lymphocytes to make antibodies and memory cells, so a later infection is destroyed rapidly.

What is the difference between active and passive immunity? Active immunity means the body makes its own antibodies and forms memory cells, giving long-term protection. Passive immunity means antibodies are given — immediate but short-lived, with no memory cells.

Why does breast milk protect a baby? It contains antibodies from the mother — an example of passive immunity.

How does cholera cause diarrhoea? Its toxin makes intestine cells secrete chloride ions, lowering the water potential, so water moves out by osmosis into the lumen.

What is herd immunity? When enough of a population is vaccinated that the pathogen cannot spread, protecting even the unvaccinated.

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Quick revision checklist

• I can define pathogen and transmissible disease
• I can give direct and indirect routes of transmission with examples
• I can list the body’s defences and how each works
• I can list methods of controlling the spread of disease
• I can give the cholera chain in full, including diarrhoea
• I write chloride, not chlorine
• I can define antigen and antibody and not confuse them
• I know lymphocytes make antibodies and phagocytes engulf
• I can explain the role of memory cells
• I can compare active and passive immunity across all five rows
• I know passive immunity produces no memory cells
• I can describe vaccination in four steps
• I know a vaccine contains a weakened pathogen, not antibodies
• I can explain herd immunity

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These notes cover topic 10 of the Cambridge IGCSE Biology (0610) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across many one-to-one IGCSE Biology lessons, with particular attention to the errors students make most often and the wording examiners reward.