Body Defence Barriers: First Lines Against Infection
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
The body has physical and chemical barriers that prevent pathogens from entering. The skin acts as a physical barrier, mucus traps pathogens in the airways, hydrochloric acid in the stomach kills bacteria in food, and blood clotting seals wounds to prevent pathogen entry.
Defence barrier questions require you to name the barrier and state how it prevents pathogen entry. Simply listing "skin, acid, mucus" without explaining the mechanism loses marks. Each barrier has a specific function you need to describe.
Physical barriers
The skin is the body's largest organ and forms a continuous physical barrier. Its outer layer of dead cells is tough and difficult for pathogens to penetrate. Sebaceous glands produce oils with antimicrobial properties.
Mucus in the nasal passages, trachea and bronchi traps inhaled pathogens and dust particles. Cilia: tiny hair-like structures on epithelial cells lining the airways: beat in a coordinated wave to sweep mucus and trapped pathogens upward toward the throat, where they are swallowed. This is the ciliary escalator.
Chemical barriers
Hydrochloric acid in gastric juice creates a strongly acidic environment (approximately pH 2) in the stomach. This denatures the enzymes and proteins of most pathogens in food and drink, killing them before they can reach the intestines.
Tears and saliva contain the enzyme lysozyme, which breaks down bacterial cell walls. These secretions continuously wash over exposed surfaces, reducing the number of bacteria that can colonise the eyes and mouth.
Clotting as a defence
When the skin is broken by a wound, the barrier is compromised. Blood clotting rapidly seals the wound: platelets aggregate, fibrin threads form a mesh, and a clot prevents further blood loss and blocks pathogens from entering through the opening.
Together, these first-line defences are non-specific: they act against all types of pathogen rather than targeting a specific one. If pathogens bypass these barriers, the immune system mounts a specific response involving white blood cells.
Try a focused question (4 marks)
Describe two ways in which the body prevents pathogens from entering through the breathing system.
Show the answer and marking guidance
Mucus in the airways traps inhaled pathogens and particles.
Cilia on the lining of the trachea and bronchi beat to sweep the mucus upward to the throat.
The trapped pathogens are swallowed and destroyed by stomach acid.
Nasal hairs filter larger particles before they enter the lower airways.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Describing stomach acid as killing all germs: some pathogens can survive acidic conditions.
Saying the skin kills pathogens rather than acting as a physical barrier to prevent entry.
Confusing non-specific barriers with the specific immune response (antibodies are not a barrier).
Forgetting to mention that cilia move mucus toward the throat for swallowing.