Describe skin and nose hairs as body defences
Explain physical barriers before discussing responses inside the body.
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall clotting and pathogen entry.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is a physical barrier?
What you will learn
- A barrier can reduce entry
- Nose hairs act at an air entrance
A barrier can reduce entry
Skin forms a physical barrier between the external environment and internal tissues. An intact outer surface makes entry harder for many pathogens; a wound can provide an entry route that was previously blocked. Protection is a biological barrier function rather than a claim that the skin surface contains no microorganisms.
The skin does not need to recognise the exact antigen of every pathogen to act as a barrier. Distinguish this general first defence from later specific antibody responses. A clot at damaged skin can help seal the entry route, linking this lesson with the earlier blood-clotting sequence.
Nose hairs act at an air entrance
Hairs in the nose help trap particles, including particles carrying pathogens, before they move deeper into the breathing passages. Their position matters: they act near an entrance, not by producing antibodies in the lungs or sterilising all inhaled air. Some particles can still pass these defences.
For a structural explanation name both the feature and its effect: intact skin limits entry into tissues, while nose hairs trap some incoming particles. Do not claim that nose hairs directly kill every pathogen. Later lessons add mucus and white-cell action, showing that several defences can cooperate rather than one being perfect.
Sources for this lesson
Pause and explain
What do nose hairs do in this syllabus description?
Worked example
A supplied wound breaks an intact skin barrier. Explain why pathogen entry may become easier.
Show the worked solution
- Skin normally separates the outside from internal tissue.
- The damage interrupts that barrier.
Answer The wound opens a route through the physical skin barrier into internal tissues.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Give one defence function of intact skin and one of nose hairs.
Give me a hint
State effects rather than only names.
Compare my reasoning
- Skin restricts entry into tissue.
- Hairs trap some incoming particles.
Skin acts as a physical barrier; nose hairs trap some particles that may carry pathogens.
Look for these in your work
- Explain skin
- Explain nose hairs
Distinguish a skin barrier from antibody specificity.
Give me a hint
One blocks entry; one recognises matching antigens.
Compare my reasoning
- Skin does not require a particular antigen match for its barrier action.
- Antibodies bind specific complementary antigens.
Skin provides a general physical entry barrier; antibody binding depends on a specific matching antigen.
Look for these in your work
- Differentiate mechanisms
- Retain specificity
A student says that microbes on skin prove the skin has no defence role. Evaluate.
Give me a hint
Surface presence and tissue entry differ.
Compare my reasoning
- Microorganisms can be on the outside.
- Intact skin can still limit their entry into internal tissues.
The claim is wrong: surface microbes do not remove the intact skin’s physical barrier function.
Look for these in your work
- Separate surface and inside
- Use the barrier mechanism
Common mistakes
- Calling skin an antibody.
- Saying nose hairs sterilise all air.
What can you explain now?
What do nose hairs do in this syllabus description?
Compare with the explanation
Trap some incoming particles
Particles may carry pathogens.
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Make it stick
Explain two structural defences and why neither requires sterile surroundings.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.