Explain how the body's defence mechanisms prevent pathogens from entering the bloodstream when a person breathes in air containing pathogens.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The respiratory tract contains several physical and chemical defences that work together to prevent pathogens from reaching the lungs and entering the bloodstream.
Model answer (4 marks)
Mucus is produced in the respiratory tract and traps inhaled pathogens, preventing them from moving further into the body.
Cilia on ciliated epithelial cells beat to move the mucus, with its trapped pathogens, upward toward the throat where it can be swallowed or expelled.
If swallowed, the mucus enters the stomach where hydrochloric acid destroys the pathogens.
These non‑specific barriers work together to stop pathogens from reaching the bloodstream and thereby prevent infection.
Cilia on ciliated epithelial cells beat to move the mucus, with its trapped pathogens, upward toward the throat where it can be swallowed or expelled.
If swallowed, the mucus enters the stomach where hydrochloric acid destroys the pathogens.
These non‑specific barriers work together to stop pathogens from reaching the bloodstream and thereby prevent infection.
Examiner tips
- Use the exact terms: mucus, cilia, hydrochloric acid, non‑specific barriers.
- Show the sequence of actions – trapping, moving, destroying – to cover all points.
- Mention that the defence is non‑specific to gain full marks.
Common mistakes
- Confusing the role of cilia with that of mucus; they are separate actions.
- Omitting the stomach’s hydrochloric acid as a defence.
- Using vague phrases like "body’s immune system" instead of the specific physical/chemical barriers.
Mark scheme (4 marks)
- Mucus is produced in the respiratory tract / airways which traps pathogens / prevents them passing further into the body
- Cilia (on ciliated epithelial cells) waft / move the mucus (containing trapped pathogens) upward / toward the throat so it can be swallowed or expelled
- Hydrochloric acid in the stomach kills pathogens that have been swallowed / destroys pathogens in the swallowed mucus
- These defences work together / are non-specific barriers to prevent pathogens reaching the bloodstream / prevent an infection developing
Key terms in this question
Related
- Revision notes: Pathogens and immunity →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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