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Cambridge IGCSE0610

Diseases and immunity

Biology 0610 Chapter Notes

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Diseases and immunity
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In this chapter

  1. 1Pathogens and transmission
  2. 2Body defences
  3. 3Controlling the spread of disease
  4. 4Antigens and antibodies
  5. 5Active immunity
  6. 6Vaccination
  7. 7Passive immunity
  8. 8Cholera

1. Pathogens and transmission

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

A pathogen is transmitted by direct contact, including through blood and other body fluids. It is transmitted indirectly from contaminated surfaces, food, animals and air.

Examiner insight

“The most common error was to give two methods of direct transmission.” “Many candidates gave examples of transmissible diseases rather than a definition.”

Worked example 11 mark

Which disease is a transmissible disease?

A coronary heart disease

B HIV infection

C rickets

D scurvy

  1. 1

    B: HIV infection. Its pathogen can be passed from one host to another. [1]

2. Body defences

The body keeps pathogens out with its skin, hairs in the nose, mucus and stomach acid.

White blood cells defend the body by phagocytosis and by producing antibodies. The body makes all five of these defences itself. Antibiotics and vaccinations are not body defences.

Examiner insight

“A common misconception was for candidates to refer to cilia in the nose rather than nasal hairs.” “Some incorrectly gave medical sources of protection such as vaccinations and antibiotics which did not answer the question.”

Worked example 11 mark

What is produced by the human body to protect it against pathogens?

antibioticsmucusred blood cells
Anonoyes
Bnoyesno
Cyesnono
Dyesyesyes
  1. 1

    B: only mucus is produced by the body to protect it against pathogens [1]

3. Controlling the spread of disease

Pathogens are transmitted indirectly from contaminated surfaces, food, animals and air. Each measure is important because it stops pathogens being passed from one host to another.

A clean water supply and sewage treatment stop pathogens being passed on in water. Hygienic food preparation, such as cooking meat at a high temperature and covering food to keep insects off, stops pathogens being passed on in food.

Good personal hygiene includes washing hands before preparing food. Waste disposal includes putting waste food in sealed containers.

Examiner insight

“Some candidates limited their answers to hygienic practices in general and not those concerned with food preparation.”

Worked example 11 mark

Good personal hygiene is important to control the spread of disease.

What is an example of good personal hygiene in the kitchen?

A cooking food at a high temperature

B disposing of waste food in sealed containers

C storing uncooked meat in a fridge

D washing hands before preparing food

  1. 1

    D: washing hands before preparing food [1]

4. Antigens and antibodies

Each pathogen has its own antigens, which have specific shapes. Lymphocytes produce antibodies.

Antibodies are proteins that bind to antigens. Specific antibodies have complementary shapes which fit specific antigens.

Binding leads to direct destruction of pathogens, or marking of pathogens for destruction by phagocytes. Phagocytes engulf pathogens. Antibodies do not.

Examiner insight

“A number of candidates described antibodies engulfing pathogens, which was not credited.” “There were good descriptions of antibodies binding to antigens, although some candidates referred to them being the same shape as antigens rather than being a complementary shape.”

Worked example 14 marks

Lymphocytes are one type of white blood cell.

Explain how lymphocytes protect the body from infection.

  1. 1

    lymphocytes produce antibodies [1]

  2. 2

    antibodies have a complementary shape to specific antigens [1]

  3. 3

    antibodies bind to antigens or pathogens [1]

  4. 4

    antibodies destroy pathogens directly, or mark them for destruction by phagocytes [1]

5. Active immunity

Active immunity is defence against a pathogen by antibody production in the body.

It is gained after an infection by a pathogen or by vaccination. In both, the person's own lymphocytes produce the antibodies. Phagocytosis is not part of active immunity.

Examiner insight

“A common error was to class phagocytosis as part of active immunity.”

Worked example 11 mark

The disease influenza is caused by a virus.

After a person is infected, ......1...... from the virus stimulate an immune response by the ......2...... which produce ......3...... .

Which row completes the description of how a person recovers from influenza?

123
Aantibodiesantigenslymphocytes
Bantibodieslymphocytesantigens
Cantigensantibodieslymphocytes
Dantigenslymphocytesantibodies
  1. 1

    D: antigens from the virus stimulate an immune response by the lymphocytes, which produce antibodies [1]

6. Vaccination

In 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.

Vaccination controls the spread of diseases. A vaccinated person has active immunity against the pathogen. As the vaccination rate in a population rises, the number of cases of the disease falls. A vaccine does not treat a person who already has the disease.

Examiner insight

“The most common misconception was that a small amount of the cholera pathogen was used, rather than a weakened or harmless form of the bacterium.” “Almost all of the candidates stated that time was needed, but often this was not linked to the production of antibodies and memory cells.”

Worked example 13 marks

Outline how vaccination gives a child long-term immunity to a disease.

  1. 1

    weakened pathogens or their antigens are put into the body [1]

  2. 2

    the antigens stimulate an immune response by lymphocytes, which produce antibodies [1]

  3. 3

    memory cells are produced, which give long-term immunity [1]

7. Passive immunity

Passive immunity is a short-term defence against a pathogen by antibodies acquired from another individual. Antibodies pass across the placenta and in breast milk.

Breast-feeding is important because breast milk gives an infant antibodies from the mother, so the infant develops passive immunity.

Memory cells are not produced in passive immunity.

Examiner insight

“The majority, however, indicated that the process involves memory cells.” “A common vague statement was to state that passive immunity was passed from a mother to an infant through breastmilk without any mention that it is antibodies that are transferred.”

Worked example 11 mark

What describes passive immunity?

A the defence against a pathogen by antibodies acquired from another individual

B the engulfing and digestion of pathogens by phagocytes

C the immunity that is gained after infection by a pathogen

D the protection against a pathogen by antibody production in the body

  1. 1

    A: defence against a pathogen by antibodies acquired from another individual [1]

8. Cholera

Cholera is a disease caused by a bacterium. It is transmitted in contaminated water.

The bacterium produces a toxin. The toxin causes secretion of chloride ions into the small intestine. This lowers the water potential in the small intestine, so water moves into the gut by osmosis. The result is diarrhoea, dehydration and loss of ions from the blood.

Examiner insight

“A common misconception was that the cholera bacteria released chloride ions or that chloride ions are toxic.”

Worked example 11 mark

Which statement explains how cholera bacteria cause diarrhoea?

A Toxins increase the concentration of chloride ions in the small intestine, decreasing the water potential so water moves into the gut by osmosis.

B Toxins increase the concentration of chloride ions in the small intestine, increasing the water potential so water moves into the gut by osmosis

C Toxins decrease the concentration of chloride ions in the small intestine, decreasing the water potential so water moves into the gut by osmosis.

D Toxins decrease the concentration of chloride ions in the small intestine, increasing the water potential so water moves into the gut by osmosis.

  1. 1

    A: more chloride ions decrease the water potential, so water moves in by osmosis [1]

Recap

  • A pathogen is a disease-causing organism.
  • A transmissible disease has a pathogen that can be passed from one host to another.
  • Body defences: skin, hairs in the nose, mucus, stomach acid and white blood cells.
  • A clean water supply, hygienic food preparation, good personal hygiene, waste disposal and sewage treatment control the spread of disease.
  • Extended: antibodies are proteins with complementary shapes that bind to specific antigens.
  • Extended: active immunity is antibody production in the body, after infection or vaccination.
  • Extended: vaccination produces memory cells, which give long-term immunity.
  • Extended: passive immunity is short-term, from acquired antibodies, with no memory cells.
  • Extended: the cholera toxin causes secretion of chloride ions, so water moves into the gut by osmosis.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State what is meant by a pathogen.1 mark
  2. A person with an infection coughs onto a door handle. Later, a second person touches the handle and becomes infected. State whether this is direct or indirect transmission. Give a reason.2 marks
  3. State two defences against pathogens found in the nose and one found in the stomach.3 marks
  4. Explain why washing hands before preparing food helps to control the spread of disease.2 marks
  5. The people of a village drink water taken from a river. Untreated sewage from the village flows into the same river. State two measures that would control the spread of disease in the village by this route. Explain why they are important.3 marks
  6. A person has antibodies against one pathogen. Explain why these antibodies do not protect against a different pathogen.2 marks
  7. A student writes: ‘Antibodies engulf pathogens.’ Explain what is wrong with this, and state how antibodies help to destroy pathogens.3 marks
  8. A child is vaccinated against a disease. Explain why the child is not protected immediately, and why the protection then lasts for a long time.4 marks
  9. A woman had measles as a child. Her breast milk contains antibodies against the measles pathogen. She breast-feeds her baby. At 9 months old the baby is given a measles vaccine. (a) Name the type of immunity the woman has, and state how she gained it. (b) Explain how breast-feeding protects the baby against measles in its first months. (c) A student says: ‘The vaccine is not needed. The baby already has its mother's memory cells, so it is immune for life.’ Explain why the student is wrong.7 marks
  10. A new drug stops the cholera toxin acting on the cells lining the small intestine. The drug does not kill the cholera bacterium. The table shows the chloride ions secreted into the small intestine and the water lost by three people. (a) Calculate the percentage decrease in the water lost per day when a person with cholera is given the drug. (b) Use the data and the idea of water potential to explain why the person given the drug loses less water. (c) The person given the drug still has cholera bacteria in the gut. Explain why sewage treatment is still important where this person lives. | person | chloride ions secreted / arbitrary units | water lost in faeces / dm³ per day | | --- | --- | --- | | no cholera | 5 | 0.2 | | cholera | 40 | 8.0 | | cholera and drug | 12 | 2.0 |8 marks

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