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Chapter 21 — Chapter 21 — Immunity
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📘Easy summary
Most diseases result from microbial infections (viruses, bacteria, fungi, protists). The immune system checks the bloodstream for foreign cells and molecules. Defence has three lines: first (non-specific) — skin, digestive tract, respiratory tract; second (non-specific) — killer cells of blood, antimicrobial proteins, inflammation, fever; third (specific) — cell-mediated and antibody-mediated immunity. Skin: epidermis 10–30 cells thick; outer stratum corneum cells are continuously shed and replaced from deep epidermis; keratin makes skin tough and impenetrable. Oil and sweat glands lower skin pH to 3–5, inhibiting microbes; sweat has lysozyme that digests bacterial cell walls. Saliva (lysozyme), stomach acids and intestinal protein-digesting enzymes kill bacteria in food. Ciliated epithelium of the nasal cavity traps bacteria; bronchi and bronchioles secrete sticky mucous that traps microbes; cilia sweep mucous upward to be swallowed.
⭐Key points
- Skin: keratin, pH 3–5, lysozyme.
- Acid and enzymes kill food bacteria.
- Mucous + cilia trap airborne microbes.
🔬Diagram
🌿Real-life example
A cut in skin breaches the first line, so microbes reach deeper tissue.
💡Exam tip
- First two lines are non-specific.
✅Quick check
1. Skin surface pH:
2. Enzyme in sweat and saliva:
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