A level-readyAll subjects · Biology · WBI14 (Unit 4)
Barriers and non-specific defences
Pathogens must get into the body before they can cause disease. Physical and chemical barriers keep most out, and non-specific responses tackle any that get through.
Ways in and barriers
Major routes of entry: the gas exchange surface (airborne pathogens such as TB), the digestive system (food and water), cuts in the skin, sexual contact, blood (needles, transfusions) and vectors (insect bites).
Barriers:
- Skin: a tough outer layer of dead, keratinised cells; sebum and sweat contain antimicrobial substances.
- Mucous membranes in the airways: mucus traps microorganisms and cilia sweep it up to the throat to be swallowed.
- Stomach acid (hydrochloric acid) kills many ingested pathogens.
- Lysozyme in tears, saliva and mucus breaks down bacterial cell walls.
- Natural flora: harmless bacteria on skin and in the gut outcompete pathogens.
- Blood clotting seals wounds.
Inflammation
When tissue is damaged or infected, mast cells release histamine:
- arterioles dilate, bringing more blood: redness and heat
- capillary walls become more permeable, so plasma, antibodies and white blood cells leave the blood: swelling
The raised temperature and the extra phagocytes help destroy pathogens.
Phagocytosis and interferon
Phagocytes (neutrophils and macrophages) are attracted to the site. A phagocyte engulfs a pathogen into a vacuole (phagosome). Lysosomes fuse with it and release hydrolytic enzymes, including lysozyme, which digest the pathogen.
Cells infected by viruses release interferon, which signals nearby cells to make proteins that stop viral replication (for example by inhibiting viral protein synthesis). It slows the spread of the virus.
Worked example
Explain why the area around an infected cut becomes red and swollen.
- Mast cells release histamine.
- Arterioles dilate, increasing blood flow: redness and warmth.
- Capillaries become more permeable, so fluid leaves the blood into the tissue: swelling.
Answer: Histamine causes vasodilation (redness) and leaky capillaries (swelling).
Check yourself
- What does lysozyme do?
- Makes antibodies
- Breaks down bacterial cell walls
- Triggers blood clotting
- Kills viruses inside cells
Show answer
Breaks down bacterial cell walls. Lysozyme hydrolyses bonds in peptidoglycan.
- Which substance released by mast cells causes inflammation?
- Interferon
- Insulin
- Lysozyme
- Histamine
Show answer
Histamine. Histamine triggers vasodilation and leaky capillaries.
- Why does inflamed tissue swell?
- Arterioles constrict
- Capillaries become more permeable, so fluid leaves the blood
- Phagocytes divide rapidly
- Red blood cells burst
Show answer
Capillaries become more permeable, so fluid leaves the blood. Extra tissue fluid causes swelling.
- What is the role of interferon?
- It digests bacteria in phagosomes
- It forms a clot
- It signals nearby cells to resist viral replication
- It is an antibody against all viruses
Show answer
It signals nearby cells to resist viral replication. Interferon slows the spread of viruses.
- How do cilia in the airways help prevent infection?
- They sweep mucus containing trapped microbes up to the throat
- They produce stomach acid
- They engulf bacteria
- They release lysozyme into the blood
Show answer
They sweep mucus containing trapped microbes up to the throat. The mucociliary escalator removes trapped pathogens.
- After a phagocyte engulfs a bacterium, what happens next?
- The phagocyte makes antibodies
- The bacterium is pushed out of the cell
- Lysosomes fuse with the phagosome and release hydrolytic enzymes
- The phagocyte divides into plasma cells
Show answer
Lysosomes fuse with the phagosome and release hydrolytic enzymes. Lysosomal enzymes digest the pathogen.
- Which barrier protects against pathogens swallowed in food?
- Hydrochloric acid in the stomach
- Keratin in the skin
- Cilia in the trachea
- Tears
Show answer
Hydrochloric acid in the stomach. Stomach acid kills many ingested microbes.
Sources
Last updated 7 November 2026. Spotted a mistake? Tell us.