Infection and Response
Pathogens, disease, immune system, vaccination, and drug development.
In this topic
Communicable Diseases and Pathogens
Pathogens are microorganisms that cause infectious disease. There are four types:
1. Bacteria: produce toxins that damage cells (e.g., Salmonella, Gonorrhoea) 2. Viruses: invade cells and reproduce inside them, destroying the cell (e.g., HIV, Measles, TMV) 3. Fungi: send out hyphae that grow and digest tissue (e.g., Rose black spot) 4. Protists: parasites that live in/on a host (e.g., Malaria — spread by mosquitoes)
Diseases spread by: direct contact, water, air (droplets), and vectors (animals that carry pathogens).
Key Points
- Bacteria and viruses are the most common pathogens
- Antibiotics kill bacteria but NOT viruses
- Viruses are much smaller than bacteria
- Vectors spread disease without being affected themselves
Example Questions
2Explain why antibiotics cannot be used to treat viral diseases.
Viruses live and reproduce inside host cells. Antibiotics target bacterial cell structures (like cell walls), which viruses don't have. Destroying the virus would mean destroying the host cell too.
[2 marks]
3Describe how the spread of Malaria can be reduced.
Use mosquito nets to prevent bites. Drain stagnant water where mosquitoes breed. Use insecticides to kill mosquitoes. Use antimalarial drugs for prevention.
[3 marks]
The Immune System
The body has several lines of defence against pathogens:
Non-specific defences (first line): - Skin: physical barrier - Nose: hairs and mucus trap particles - Trachea and bronchi: mucus traps pathogens, cilia waft mucus to the throat - Stomach: hydrochloric acid kills most pathogens
Specific immune response (white blood cells): 1. Phagocytosis: phagocytes engulf and digest pathogens 2. Antibody production: lymphocytes produce specific antibodies that lock onto antigens 3. Antitoxin production: neutralise toxins produced by bacteria
Vaccination: injecting a dead or weakened form of the pathogen stimulates antibody production. Memory cells remain, so a faster response occurs if exposed again.
Key Points
- Antigens are unique proteins on the surface of pathogens
- Antibodies are specific to particular antigens
- Memory cells provide long-term immunity
- Herd immunity protects those who can't be vaccinated
Example Questions
4Explain how vaccination prevents disease.
A dead/weakened pathogen is injected. White blood cells recognise the antigens and produce specific antibodies. Memory cells are created. If the person is exposed to the real pathogen, memory cells rapidly produce antibodies before the person becomes ill.
[4 marks]