Diseases and Immunity Notes
Cambridge IGCSE Biology: Revision notes
Key facts
- A pathogen is a disease-causing organism; a transmissible disease passes from host to host.
- First defences: skin, nasal hairs, mucus and stomach acid. Second: phagocytes and lymphocytes.
- Antibodies have a shape complementary to one antigen; memory cells give long-term active immunity.
- Vaccines contain weakened pathogens or antigens, not antibodies.
- Passive immunity (placenta, breast milk) is immediate but short-lived, with no memory cells.
- Cholera toxin makes the gut secrete chloride ions, so water follows by osmosis.
Pathogens and spread
Pathogens spread by direct contact, indirect contact, animal vectors or the air, and each route needs a different control.
A pathogen is a disease-causing organism such as a bacterium, virus, fungus or protoctist. A transmissible disease is one where the pathogen passes from an infected host to another host.
Direct contact
- Through blood, body fluids or touching
Indirect contact
- Contaminated objects, food, water or soil
Animal vectors
- An animal carries the pathogen, e.g. malaria by mosquitoes
Airborne
- Droplets breathed in, e.g. influenza
Malaria is spread by mosquitoes. Which transmission route is this?
Natural defences
Barriers keep pathogens out, and white blood cells deal with any that get in.
The skin and mucous membranes are the first line of defence. Hairs in the nose filter air. Mucus traps pathogens and contains antimicrobial enzymes. Stomach acid (about pH 2) denatures pathogen proteins. Inside the body, phagocytes engulf pathogens and lymphocytes make antibodies.
Skin
- Physical barrier
Nasal hairs
- Filter inhaled particles
Mucus
- Traps pathogens
Stomach acid
- Denatures pathogen proteins
Which defence kills bacteria in swallowed food?
Active immunity
The body makes its own antibodies and memory cells, giving long-term protection against a specific pathogen.
Each pathogen has antigens on its surface. Lymphocytes make antibodies with a complementary shape that bind the antigen, destroying the pathogen or marking it for phagocytes. Memory cells remain, so the response is fast next time.
Active immunity comes from infection or vaccination.
- 1
Antigen detected
lymphocytes recognise the pathogen
- 2
Antibodies made
specific to that antigen
- 3
Pathogen destroyed
or marked for phagocytes
- 4
Memory cells remain
fast response next time
What does a vaccine contain?
Passive immunity
Passive immunity gives borrowed antibodies: instant but short-lived, with no memory cells.
Passive immunity uses antibodies from another individual: across the placenta, in breast milk, or by injection. Infants cannot yet make enough antibodies, so breast milk protects them in early life.
| Active | Passive | |
|---|---|---|
| Source | Body makes antibodies | Antibodies from another person |
| How acquired | Infection or vaccination | Placenta, breast milk, injection |
| Speed | Slower | Immediate |
| Duration | Long-term | Short-term |
| Memory | Memory cells: yes | Memory cells: no |
Active
- Source:
- Body makes antibodies
- How acquired:
- Infection or vaccination
- Speed:
- Slower
- Duration:
- Long-term
- Memory:
- Memory cells: yes
Passive
- Source:
- Antibodies from another person
- How acquired:
- Placenta, breast milk, injection
- Speed:
- Immediate
- Duration:
- Short-term
- Memory:
- Memory cells: no
Why does passive immunity not last?
Cholera and prevention
Cholera toxin makes the gut secrete chloride ions, so water follows by osmosis; clean water prevents it.
Cholera is caused by the bacterium Vibrio cholerae, spread in contaminated water. Its toxin makes the small intestine secrete chloride ions (). Water moves in by osmosis, causing severe diarrhoea and loss of water and ions.
Treat with oral rehydration therapy. Prevent with clean water, sewage treatment, hygiene and safe waste disposal.
- 1
Ingestion
bacteria swallowed in contaminated water
- 2
Toxin made
in the small intestine
- 3
Ions secreted
chloride ions enter the gut
- 4
Osmosis
water moves into the gut
- 5
Diarrhoea
loss of water and ions can cause shock
Why does water move into the gut in cholera?
Try an exam question
Explain how cholera bacteria cause diarrhoea, and how the spread of cholera can be prevented.
[5 marks]
- [1]The bacterium produces a toxin in the small intestine.
- [1]The toxin causes chloride ions to be secreted into the gut.
- [1]This lowers the water potential in the gut, so water moves in by osmosis.
- [1]This causes watery diarrhoea.
- [1]Prevented by clean water supply and sewage treatment, which remove the route of transmission.
That's the notes covered.
Carry on to the next subtopic.