Disease and body defences
Types of pathogens
A pathogen is a microorganism that causes disease. The main types are:
| Pathogen | Type | Example disease | Treated by |
|---|---|---|---|
| Bacteria | Prokaryote | Tuberculosis, cholera, food poisoning | Antibiotics |
| Virus | Non-cellular | Influenza, HIV, measles | Antivirals / vaccines (antibiotics do NOT work) |
| Fungi | Eukaryote | Athlete's foot, ringworm | Antifungals |
| Protist | Eukaryote | Malaria (Plasmodium) | Antimalarials |
How pathogens cause disease:
- Bacteria: reproduce rapidly, produce toxins that damage tissues.
- Viruses: invade host cells, hijack cell machinery to replicate, causing cell destruction.
Spread of disease (transmission)
- Direct contact: touching, sexual contact (HIV, STIs).
- Droplets: coughing, sneezing (influenza, COVID-19).
- Contaminated food/water: cholera, Salmonella.
- Vectors: mosquitoes carry Plasmodium (malaria); fleas carry Yersinia pestis (plague).
- Blood: HIV, hepatitis via shared needles.
The body's defences
Non-specific (first and second line)
- Skin: physical barrier; sebaceous glands secrete sebum (slightly acidic — inhibits growth).
- Mucous membranes: mucus traps pathogens in airways; cilia sweep mucus to throat.
- Stomach acid: pH 1–2 kills most swallowed pathogens.
- Inflammation: increased blood flow; phagocytes arrive.
- Phagocytosis: phagocytes (neutrophils, macrophages) engulf and digest pathogens — non-specific.
Specific (third line) — the immune response
- Pathogen enters body; carries antigens (proteins on its surface — unique marker).
- Lymphocytes (B-cells) recognise the specific antigen.
- B-cells multiply and produce antibodies — complementary in shape to the antigen.
- Antibodies bind to antigens → agglutination (clumping) → easier for phagocytes to destroy.
- Memory cells remain — if same pathogen encountered again, faster, larger antibody response → immunity.
Vaccines
A vaccine contains weakened/dead pathogens or their antigens. It stimulates the immune system to produce antibodies and memory cells without causing disease. If the real pathogen enters later, memory cells enable a rapid response.
Herd immunity: if enough of a population is vaccinated, the pathogen cannot spread easily — protects those who cannot be vaccinated.
Antibiotics
Antibiotics kill or inhibit the growth of bacteria only. They work by:
- Disrupting cell wall synthesis (e.g. penicillin)
- Inhibiting protein synthesis
- Inhibiting DNA replication
They do NOT work on viruses (viruses have no cell wall; they use host cell machinery).
Antibiotic resistance: overuse/misuse of antibiotics selects for resistant strains (MRSA, MRSA). Prevention: complete courses; don't use for viral infections; reduce agricultural use.
⚠Common mistakes
- Saying antibiotics treat viral infections — they do not.
- Confusing antigens and antibodies — antigen is on the pathogen (A = "Alien" marker); antibody is produced by the host B-cell.
- Saying the vaccine contains the actual disease — vaccines use weakened/dead pathogens or just the antigens.
- Forgetting memory cells are the key to long-lasting immunity after vaccination.
AI-generated · claude-opus-4-7 · v3-ccea-biology