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Topic 10 · 10.1

Diseases and immunity

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Describe a pathogen and a transmissible disease, and how pathogens are transmitted directly and indirectly.
  • Describe the body’s defences and explain how hygiene, clean water, waste disposal and sewage treatment control the spread of disease.
  • Describe active immunity, antigens and antibodies, and vaccination EXTENDED.
  • Explain passive immunity and breast-feeding; describe how cholera causes diarrhoea EXTENDED.

📚The biology

Pathogens and how they spread

A pathogen is a disease-causing organism (bacteria, viruses, fungi, protoctists). A transmissible disease is one in which the pathogen can be passed from one host to another. Pathogens are transmitted:

  • directly: by direct contact, including through blood and other body fluids (e.g. HIV);
  • indirectly: from contaminated surfaces, food, animals (such as mosquitoes) and air (droplets from coughs and sneezes).

The body’s defences

An outline of a human body with labelled defences: skin, a barrier to pathogens; hairs in the nose trap particles; mucus in the airways traps pathogens; stomach acid kills pathogens in food; white blood cells in the blood carry out phagocytosis and make antibodies.
The first four keep pathogens out; white blood cells deal with any that get in.
Three stages of phagocytosis: a phagocyte moves towards a pathogen; its cytoplasm flows around and engulfs it; enzymes digest the pathogen inside a vacuole.
Phagocytosis: engulfing and digesting pathogens.

Controlling the spread of disease

clean water supply: removes pathogens spread in water, such as the cholera bacterium
hygienic food preparation: cooking thoroughly, keeping raw meat apart, storing food cold
good personal hygiene: washing hands, especially after using the toilet
waste disposal: waste in sealed bins does not attract flies and rats that carry pathogens
sewage treatment: removes pathogens from human waste before water is returned to rivers

Antibodies and active immunity EXTENDED

Pathogen A has triangle-shaped antigens and pathogen B circle-shaped antigens. Y-shaped antibodies made against A have ends that fit the triangles; they bind to pathogen A's antigens, making pathogens clump together or marking them for phagocytes to destroy.
EXTENDED Antibodies against A do not fit B’s antigens: each antibody is specific.
  • EXTENDED Each pathogen has its own antigens, molecules with specific shapes on its surface.
  • Antibodies are proteins that bind to antigens, leading to direct destruction of pathogens or marking them for destruction by phagocytes. Specific antibodies have complementary shapes that fit specific antigens.
  • Active immunity is defence against a pathogen by antibody production in the body. It is gained after an infection or by vaccination.

EXTENDED Vaccination: weakened pathogens or their antigens are put into the body; the antigens stimulate an immune response by lymphocytes, which produce antibodies; memory cells are produced that give long-term immunity. If the real pathogen arrives later, memory cells make antibodies so fast that the person does not become ill. When most of a population is vaccinated, the pathogen cannot spread easily, which also protects people who cannot be vaccinated.

A graph of antibody concentration against time. After vaccination there is a slow, small rise that stays below the level needed to prevent illness, then falls. After a later infection there is a very fast, large rise to far above that level, because of memory cells.
EXTENDED The second response is faster and larger because memory cells already exist. (Schematic.)

EXTENDED Passive immunity is short-term defence by antibodies acquired from another individual: across the placenta before birth, and in breast milk. No memory cells are produced, so it does not last. Breast-feeding is important because a newborn’s immune system is not yet fully developed; antibodies in milk protect it while it builds its own.

Cholera EXTENDED

The lining of the small intestine. Cholera bacteria in the lumen release a toxin that makes the cells secrete chloride ions into the gut; water follows from the blood by osmosis. The result is watery diarrhoea, dehydration and loss of ions from the blood.
EXTENDED The chloride ions lower the water potential in the gut, so water moves in by osmosis.

EXTENDED Cholera is caused by a bacterium transmitted in contaminated water. The bacterium produces a toxin that causes secretion of chloride ions into the small intestine, causing osmotic movement of water into the gut, which causes diarrhoea, dehydration and loss of ions from the blood.

✏️Worked example EXTENDED

Use the immune response graph above. (a) Describe two differences between the response after vaccination and the response after the later infection. [2] (b) Explain these differences. [2] (c) A baby gets antibodies against measles in breast milk. Explain why this protection does not last. [2]

(a) After infection, antibody concentration rises much faster, and reaches a much higher peak (well above the level that prevents illness).

(b) Vaccination produced memory cells. When the same antigen appears again, memory cells recognise it and lymphocytes produce large amounts of the specific antibody quickly.

(c) This is passive immunity: the antibodies came from the mother and are broken down over time; the baby’s own lymphocytes have not been stimulated, so no memory cells are made.

Check it. Active immunity = the body makes its own antibodies and memory cells (long-term). Passive = antibodies received (short-term).
“The vaccine contains antibodies.” A vaccine contains weakened pathogens or their antigens; the body makes the antibodies.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Multiple choice.) What is a transmissible disease? A: any disease caused by poor diet. B: a disease in which the pathogen can be passed from one host to another. C: a disease that is inherited. D: a disease caused by smoking.
B.
2. (Theory.) Give one example each of direct and indirect transmission of a pathogen. [2]
Direct: contact with infected blood or body fluids (e.g. HIV). Indirect: any of contaminated food or water, surfaces, animals (e.g. mosquitoes), air (droplets).
3. (Theory.) Explain how mucus and hairs in the nose help to prevent infection. [2]
Hairs in the nose trap larger particles; sticky mucus traps pathogens and dust so they do not reach the lungs.
4. (Theory.) Explain why treating sewage reduces the spread of disease. [2]
Sewage contains pathogens from human waste; treatment removes them, so they do not contaminate rivers and drinking water.
5. (Theory.) EXTENDED Explain why antibodies against one pathogen do not protect against a different pathogen. [2]
Each pathogen has antigens of a specific shape; an antibody has a complementary shape that fits only those antigens.
6. (Theory.) EXTENDED Outline the process of vaccination. [3]
Weakened pathogens or their antigens are put into the body; the antigens stimulate lymphocytes to produce antibodies; memory cells are produced, giving long-term immunity.
7. (Theory.) EXTENDED Explain why vaccinating most of a population controls the spread of a disease. [2]
Most people are immune, so the pathogen cannot be passed from host to host; fewer people are infected, which also protects those who are not vaccinated.
8. (Theory.) EXTENDED Explain why a person with cholera is given a drink containing water and ions. [2]
Cholera causes loss of water and ions in diarrhoea; the drink replaces them, treating dehydration and restoring the ions in the blood.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • World Health Organization — fact sheets on cholera and on vaccines
  • British Society for Immunology — how vaccines work, with animations