Human Defences Against Disease
Non-Specific Defences
The body has several non-specific defences that act as barriers to prevent pathogens from entering. These work against all pathogens rather than targeting specific ones.
Physical and Chemical Barriers
| Defence | How it works |
|---|---|
| Skin | Acts as a physical barrier; the outer layer of dead cells is tough for pathogens to penetrate; if cut, blood clots quickly to seal the wound |
| Nose hairs and mucus | Trap particles and pathogens in inhaled air; mucus is sticky and contains lysozyme (an enzyme that destroys bacterial cell walls) |
| Cilia | Tiny hair-like structures lining the airways; sweep mucus containing trapped pathogens up to the throat, where it is swallowed |
| Stomach acid | Hydrochloric acid (pH ~2) kills most pathogens that are swallowed in food or mucus |
| Tears and saliva | Contain lysozyme enzymes that break down bacterial cell walls |
| Trachea and bronchi | Secrete mucus to trap pathogens; have cilia to move mucus upward |
The Immune System — Specific Defence
If pathogens get past the non-specific defences, the immune system responds. The key players are white blood cells (leucocytes).
White Blood Cell Responses
White blood cells defend the body in three main ways:
1. Phagocytosis
- Phagocytes (e.g. neutrophils, macrophages) detect pathogens and move towards them
- The phagocyte engulfs (surrounds) the pathogen, taking it inside the cell in a vacuole
- Enzymes inside the phagocyte digest and destroy the pathogen
- This is a non-specific response — phagocytes engulf any foreign material
2. Antibody production
- Lymphocytes (B-lymphocytes) recognise specific antigens on the surface of pathogens
- Antigens are unique protein markers — each pathogen has different antigens
- The lymphocyte produces antibodies that are complementary to the specific antigen (like a lock and key)
- Antibodies attach to the pathogen, causing them to clump together (agglutination), making it easier for phagocytes to engulf them
- Antibodies can also neutralise toxins or mark pathogens for destruction
- Each type of antibody is specific — it only works on one type of antigen
3. Antitoxin production
- Some white blood cells produce antitoxins
- These neutralise the toxins produced by bacteria
- This does not kill the pathogen but counteracts the harm caused by its toxins
Immune Memory
After an infection:
- The lymphocytes that produced the correct antibodies become memory cells
- If the same pathogen enters the body again, memory cells recognise its antigens immediately
- Antibodies are produced much faster and in larger quantities the second time
- The person does not become ill — they are immune to that disease
- This is the basis of vaccination
Exam Tips
- Phagocytosis is non-specific (works on any pathogen); antibody production is specific (one antibody for one antigen)
- Always use the term antigen for the marker on the pathogen, and antibody for the protein produced by lymphocytes
- White blood cells do NOT "eat" pathogens — use the correct term: engulf or phagocytose
- Antibodies are specific to particular antigens — always state this in exam answers, often worth a mark on its own