IB Biology HLDisease & ImmunityPaper 1 & 2~10 min read
Pathogen Barriers: Skin & Mucous Membranes
Almost every pathogen that reaches you never gets inside. Your skin and your mucous membranes are the first line of defence, and they work without any immune cells being involved at all — which is exactly what makes them a “primary” defence.
📘 What you need to know
Skin and mucous membranes form the primary defence against pathogens.
Skin is a tough physical barrier. Cuts are sealed by blood clots so pathogens cannot enter.
Sebum from sebaceous glands in the hair follicles keeps the skin at a low pH, which stops microorganisms growing.
Mucous membranes line the vulnerable openings: the airways, the areas around the reproductive organs, and the digestive system.
Goblet cells produce mucus containing glycoproteins, which traps microorganisms and particles.
Cilia of the ciliated epithelium sweep the mucus upwards, where it is swallowed and destroyed by stomach acid, or expelled.
Mucus also contains lysozyme enzymes, which have antibacterial properties.
Why we call it a primary defence
These barriers are non–specific. They do not identify what is landing on them, and they do not get better with practice. They block everything the same way, all the time. That is the opposite of the adaptive immune system you will meet in a few pages.
The idea in one line
Primary defence = keep the pathogen out. Immune response = deal with the ones that got in.
Skin
Skin is the largest organ in the body, and it is covered in microorganisms. Almost all of them cause no trouble at all, simply because they cannot get through. Skin defends you in three separate ways.
Sebum is the part students forget. It is not there to moisturise you — it holds the skin surface at a pH that most microorganisms cannot grow in.
The three jobs, in order
A tough physical barrier. Dead, keratin–filled cells packed tightly together give pathogens nothing to cross and nothing to feed on.
Blood clots. When the skin is cut, the barrier is broken. Clotting seals the gap so pathogens cannot get in through the wound.
Chemical protection. Sebaceous glands in the hair follicles produce sebum, which maintains a low skin pH and inhibits the growth of microorganisms.
Examiners like the phrase “low pH inhibits the growth of microorganisms“. Writing “sebum kills germs” is too vague and often gets nothing. Sebum makes the surface an unpleasant place to grow, which is not quite the same as killing.
Mucous membranes
Skin cannot cover everything. You have to breathe, eat and reproduce, so there are openings, and every opening is a possible route in. Those routes are lined with mucous membranes: the airways, the areas around the reproductive organs (foreskin and vagina), and the digestive system.
A mucous membrane is not just a wet surface. It is an active trap.
Think of it as a conveyor belt with glue on it. Anything that lands gets stuck, then carried out of the airway and swallowed.
The sequence, step by step
🧩 How a mucous membrane clears a pathogen
Goblet cells in the membrane produce mucus containing glycoproteins, which makes it thick and sticky.
Microorganisms and dust particles land on the surface and become trapped in the mucus.
The cilia of the ciliated epithelium beat and sweep the mucus upwards away from the lungs.
The mucus reaches the throat and is swallowed, so the trapped microorganisms are destroyed by stomach acid. Some is coughed out instead.
Along the way, lysozyme enzymes in the mucus attack bacteria directly.
Why smoking matters here. Smoke paralyses and destroys cilia. The mucus is still made but it can no longer be moved, so it sits in the airways with everything it has trapped. That is a large part of why smokers get more chest infections.
Part
What it is
What it does
Goblet cell
A cup–shaped cell in the epithelium
Makes and releases mucus containing glycoproteins
Mucus
A thick, sticky glycoprotein solution
Traps microorganisms and particles before they reach cells
Cilia
Tiny hair–like structures on the epithelial cells
Beat to sweep the mucus upwards to be swallowed
Lysozyme
An enzyme found in mucus (and in tears and saliva)
Antibacterial — damages bacteria caught in the mucus
Sebum
An oily secretion from sebaceous glands
Maintains a low skin pH that inhibits microbial growth
Worked examples
WE 1
Outline how skin prevents infection
Outline three ways in which the skin acts as a barrier to pathogens. (3 marks)
Point 1: physical
Skin is a tough physical barrier that pathogens cannot cross while it is intact.
Point 2: repair
If the skin is cut, blood clots form and seal the wound, closing the entry point.
Point 3: chemical
Sebaceous glands in the hair follicles produce sebum, which keeps the skin at a low pH and inhibits the growth of microorganisms.
Physical barrier, clotting, low pH from sebumthree marks means three separate mechanisms — do not write the same idea three ways
WE 2
Explain how mucous membranes protect the airways
Explain how the ciliated epithelium of the airways reduces the risk of infection. (4 marks)
Point 1: making the mucus
Goblet cells in the epithelium produce mucus containing glycoproteins.
Point 2: trapping
Microorganisms and particles that are breathed in become trapped in the sticky mucus, so they never reach the lung tissue.
Point 3: moving it
The cilia beat and sweep the mucus upwards towards the throat.
Point 4: destroying it
The mucus is swallowed and the trapped microorganisms are destroyed by stomach acid, or the mucus is expelled. Lysozyme enzymes in the mucus are also antibacterial.
Trap in mucus, sweep with cilia, swallow, destroy with acidthe cilia move the mucus — they do not move the pathogen directly
WE 3
Apply it: damaged cilia
A patient has an illness that stops their cilia beating normally. Suggest why they suffer repeated chest infections. (2 marks)
Point 1: what stops working
Mucus is still produced and still traps microorganisms, but it can no longer be swept upwards out of the airways.
Point 2: the consequence
Mucus containing trapped pathogens builds up in the lungs, giving those pathogens time to reproduce and cause infection.
Trapping still works, removal does not, so pathogens stay“suggest” means apply what you know — start from which step of the sequence has failed
💡 Exam tips
Call them primary defences and say they are non–specific.
Use the exact chain: goblet cell → mucus → trapped → cilia → swallowed → stomach acid.
Say sebum keeps a low pH that inhibits growth — not that it “kills germs”.
Name lysozyme as the antibacterial enzyme in mucus. It is a cheap extra mark.
Remember the three mucous membrane sites: airways, reproductive organs, digestive system.
If a question mentions a cut, burn or wound, the expected answer is that the barrier has been broken.
⚠ Common mistakes
Saying cilia trap the pathogens. The mucus traps them; the cilia move the mucus.
Calling skin part of the immune response. It is a barrier that works before any immune response starts.
Writing “mucus” when you mean “mucous”. Mucus is the substance; mucous is the adjective, as in mucous membrane.
Forgetting the swallowing step. Sweeping the mucus up is only half the story — stomach acid finishes it.
Saying skin has no microorganisms on it. It is covered in them. They just cannot get in.
Confusing sebum with sweat. Sebum is the oily secretion from sebaceous glands in hair follicles.
Up next: Blood Clotting. We just said a clot seals a cut and keeps pathogens out. The next page shows exactly how that plug is built, and why one tiny signal can produce a full clot in seconds.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
