GCSE Biology Infection & Response Notes
Pathogens cause communicable disease. The immune system fights them; vaccines and antibiotics support that defence.
Key concepts
- Pathogen
- A disease-causing microorganism: bacterium, virus, fungus or protist.
- Immune response
- White blood cells engulf pathogens (phagocytosis), produce antibodies specific to antigens, and release antitoxins.
- Vaccination
- Introducing a small quantity of dead/inactive pathogen so the immune system makes memory cells and antibodies — future infection is destroyed quickly.
- Antibiotic
- A drug that kills bacteria inside the body. Ineffective against viruses.
- Skin, stomach acid, mucus and cilia are the body's first-line barriers.
- Herd immunity protects those who cannot be vaccinated when a high percentage of the population is immune.
- Overuse of antibiotics has driven resistance (e.g. MRSA).
- New drugs go through preclinical (cells/animals) then clinical (human) trials — testing toxicity, efficacy and dose.
Worked example
Explain how vaccination protects a person from measles.
- 1A weakened/inactive form of the measles virus is injected.
- 2White blood cells produce antibodies specific to its antigens.
- 3Memory cells remain in the blood.
- 4If the real virus is met, antibodies are made quickly and in large quantity, destroying it before symptoms appear.
Answer: The vaccine triggers memory cell production so any future measles infection is destroyed rapidly.
Common mistakes
✗ Saying antibiotics kill viruses.
✓ Antibiotics only work against bacteria; viral infections need antivirals or vaccination.
✗ Confusing antigen and antibody.
✓ Antigen = marker on the pathogen; antibody = Y-shaped protein made by lymphocytes to match the antigen.
✗ Missing the role of memory cells.
✓ Memory cells are why a vaccinated person responds more quickly on a second exposure.
Quick check
Try these, then reveal the answers.
Name the four types of pathogen.Reveal answer
Bacteria, viruses, fungi and protists.
Why do doctors avoid over-prescribing antibiotics?Reveal answer
To reduce the spread of antibiotic-resistant bacteria.
What are the two stages of drug testing after lab work?Reveal answer
Preclinical trials (cells/tissues/animals) and clinical trials (humans).
Exam tips
- Use precise words: 'antigens', 'antibodies', 'memory cells', 'lymphocytes' and 'phagocytes'.
- For antibiotic resistance, always mention 'random mutation → survival advantage → passed on'.
- When discussing vaccination, cover both individual and population (herd) protection.
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Frequently asked questions
What is the difference between a bacterium and a virus?+
Bacteria are living cells that reproduce by dividing; viruses are non-living particles that reproduce inside host cells and burst them.
Why do vaccines contain weakened pathogens?+
So they trigger an immune response without causing the disease, allowing memory cells to form for rapid future defence.
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