A barrier defence blocks or destroys pathogens at the point where they try to enter the body. It works before the immune system has to do anything and acts in the same way against any pathogen. In questions you are usually asked to name the defence and explain how it works.
This lesson follows on from transmission and symptoms and sits inside disease and defence concepts.
Which barrier defences should you know?
- Skin: a tough outer layer that pathogens cannot easily pass through while it is unbroken.
- Mucus in the airways: a sticky layer that traps pathogens and dust in the breathing passages.
- Cilia: tiny hairs on the lining cells that move the mucus and trapped pathogens away from the lungs.
- Stomach acid: hydrochloric acid gives the stomach a very low pH, and this kills many pathogens that were swallowed with food or mucus.
Your syllabus may phrase these slightly differently, so check the Cambridge page for your exam year.
How do you write an explanation, not a list?
Use a three-part sentence: what it is, what it does, what happens to the pathogen. For example, “Mucus is sticky, so pathogens are trapped in it.”
The word “so” is the link. It turns a label into an explanation, and the extra clause is where the marks sit.
Worked example
Question: A person breathes in air containing bacteria. Describe how the body’s barrier defences reduce the chance of the bacteria reaching the lungs, and what happens in the stomach if the bacteria are swallowed.
Step 1, trapping: the airway lining makes mucus, which is sticky, so bacteria stick to it and are trapped.
Step 2, removal: cilia on the lining cells move the mucus upwards, away from the lungs, so the trapped bacteria are carried out.
Step 3, swallowing: if the mucus is swallowed, it enters the stomach.
Step 4, the acid: the stomach contains hydrochloric acid, which gives a very low pH, and this kills many of the bacteria.
Answer: mucus traps the bacteria, cilia sweep the mucus away from the lungs, and stomach acid kills many that are swallowed.
The mistake to watch for
A common slip is to give the wrong action to the wrong structure.
Mistaken answer: “Cilia trap the bacteria and mucus kills them.”
The student knew both words but swapped their jobs.
The correction is to attach one verb to each structure and stick to it. Mucus traps, cilia sweep and acid kills.
Keeping those verbs fixed prevents the swap.
Check yourself
Try these, then open each answer.
1. Explain why a deep cut increases the chance of infection.
Show answer
The skin is a barrier that stops pathogens entering. A cut breaks the barrier, so pathogens can pass through into the tissue beneath.
2. State the job of cilia in the airways.
Show answer
Cilia move the mucus, with the pathogens trapped in it, away from the lungs.
3. Why do many swallowed pathogens fail to survive in the stomach?
Show answer
The stomach contains hydrochloric acid, which gives a very low pH. The acid kills many pathogens.
Where this leads next
When a pathogen gets past these barriers, a different kind of defence begins. Move on to modelling an immune response at syllabus level, then check yourself with the mixed practice set.
Students who know the list but lose marks on the wording can work on that one sentence at a time in online one-to-one Biology tuition.