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Cambridge IGCSE Biology · 0610

Chapter 10: Diseases and immunity

Pathogens and transmission

Pathogen
An organism that causes disease, including bacteria and viruses.
Host
An organism that harbours these pathogens.
Transmissible disease
A disease in which the pathogen can be passed from one host to another.
Methods of transmission
  1. Direct contact: The pathogen is transferred from host to host via blood and other body fluids.
  2. Indirectly: The pathogen is transferred from contaminated surfaces, foods, animals, and air.

Exam Traps

  • Do not confuse the host (harbours the pathogen) with the pathogen itself.
  • Avoid saying all diseases are transmissible — only those where the pathogen spreads between hosts.

Body defences

The body’s first line of defence attempts to prevent pathogens from entering the body.

  1. Mechanical barriers: Includes hairs in the nose and the skin.
  2. Chemical barriers: Includes mucus, stomach acid, and tears.
  3. Cells: Once a pathogen infects the body, an immune response occurs involving white blood cells (phagocytosis and antibody production).

Exam Traps

  • Avoid saying antibodies are produced by phagocytes; lymphocytes make antibodies.

Active immunity and the immune response

Active immunity
A defence against a pathogen by antibody production in the body. It is gained after an infection or through vaccination.
Antigens
Each pathogen has specific antigen proteins with specific shapes on its cell membrane.
Antibodies
Proteins produced by lymphocytes. They have complementary shapes that only fit specific antigens.
Mechanism
Antibodies bind to antigens to produce an antibody-antigen complex. This causes pathogens to clump together, making them harmless or marking them for destruction by phagocytes.
Memory cells
After a pathogen is killed, some lymphocytes remain as memory cells. If the same pathogen enters again, they recognise the antigens and produce antibodies more quickly, providing long-term immunity.
Antibody-antigen binding showing Y-shaped antibodies with complementary shapes binding to antigens on a pathogen
Diagram 1: Antibody-antigen binding. A pathogen with specifically shaped antigens on its surface. Y-shaped antibodies with a complementary shape bind to the antigens to form a complex.

Exam Traps

  • Do not say antibodies have identical shapes to antigens — they are complementary, not identical.
  • Avoid claiming active immunity is short-term; passive immunity is short-term because no memory cells form.

Vaccination and herd immunity

  1. A dead or attenuated (weakened) version of a pathogen or its antigens is given to the patient.
  2. The antigens stimulate an immune response by lymphocytes, which produce antibodies.
  3. Memory cells are produced, providing long-term immunity.
Herd immunity
This occurs when a large proportion of a population is vaccinated and immune. This prevents the disease from spreading, thereby protecting the few individuals who cannot be vaccinated for medical reasons.

Exam Traps

  • Do not say live virulent pathogens are injected — vaccines use dead or weakened forms.
  • Avoid confusing vaccination (active immunity) with injecting ready-made donor antibodies (passive immunity).

Passive immunity

Passive immunity
A short-term defence against a pathogen gained by acquiring antibodies from another individual.
Examples
  • Antibodies passing from a mother to a baby across the placenta or through breast milk.
  • Injections of antibodies from a donor.
Key feature
It is short-term because memory cells are not produced.

Exam Traps

  • Do not say memory cells are produced in passive immunity — that is why protection is short-term.
  • Avoid describing passive immunity as long-term or gained after infection by the recipient.

Cholera

Cause
Infection by Vibrio cholerae bacteria, transmitted in contaminated water.
Mechanism of action
  1. The bacteria reach the small intestine, bind to the wall, and produce a toxin.
  2. The toxin causes cells lining the small intestine to secrete chloride ions into the lumen.
  3. This increases the concentration of ions in the lumen, decreasing the water potential.
  4. Water moves from the cells into the lumen by osmosis.
  5. This leads to the loss of large amounts of water via faeces, causing diarrhoea, dehydration, and loss of ions from the blood.
Osmotic movement in cholera showing chloride ions pumped into the small intestine lumen and water moving out of cells
Diagram 2: Osmotic movement in cholera. The small intestine wall shows chloride ions being pumped into the lumen. Arrows show water moving out of the cells into the lumen due to the water potential gradient.

Exam Traps

  • Do not say water potential increases in the lumen — Cl− secretion lowers it.

Controlling the spread of disease

To prevent pathogens from spreading, it is important to maintain:

  1. Clean water supply.
  2. Hygienic food preparation.
  3. Good personal hygiene.
  4. Waste disposal and sewage treatment.

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