First Line of Defence
You'll be able to classify a given defence as a physical or chemical barrier.
Researchers at the University of Melbourne reported in 2021 that your skin, roughly 2 m² of living tissue, repels an estimated 10 million pathogen contacts every single day before a single white blood cell needs to act.
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Q1 · What do you already know about how your body protects itself from germs and other harmful invaders?
Q2 · If someone suffered a severe burn that removed a large area of skin, why do you think they might become more vulnerable to infections than someone with healthy skin?
● Know
- Identify the physical barriers of the first line of defence, including skin, mucous membranes, and cilia
- Name the chemical barriers, including tears, saliva, lysozyme, stomach acid, and sebum
- Recognise that the first line of defence is non-specific and acts before infection takes hold
● Understand
- Explain how each physical barrier physically blocks or traps pathogens
- Describe how chemical barriers kill or weaken pathogens before they enter tissues
- Relate a break in the barriers, such as a cut or burn, to an increased risk of infection
● Can do
- Classify a given defence as a physical or chemical barrier
- Predict how damage to a barrier affects the risk of infection
- Explain how everyday actions help the first line of defence keep working
Use this quick check to connect the first-line barriers in this lesson. Can you identify a barrier, explain whether it is physical or chemical, and describe how it blocks, traps, sweeps away or weakens pathogens before they enter tissues?
Use your answers to find a gap in your understanding of skin, mucus, cilia or chemical barriers. Then choose one barrier and explain how damage or reduced function could make infection more likely.
Text description
- Tears and saliva contain the enzyme lysozyme.
- Mucous membranes trap pathogens (nose, gut).
- Cilia in the airways sweep mucus up.
- Skin is a waterproof keratin layer.
- Sebum is an antimicrobial oil.
- Stomach acid, pH 1.5–3.5, kills microbes.
- All first-line defences are non-specific.
When washing hands, soap and running water help remove pathogens from the skin surface. This supports the first line of defence by reducing the number of microbes that could enter through the eyes, nose, mouth or a break in the skin.
Check your understanding by teaching someone how one barrier works. For example, explain how mucus traps particles and how cilia move that mucus out of the airways. Naming a barrier is a start; describing what it does shows how it protects the body.
- Skin
- Mucus
- Cilia
- Lysozyme
- Stomach acid
- Creates an acidic environment that destroys many swallowed microbes
- Sticky substance that traps pathogens and particles
- Enzyme in tears and saliva that damages bacterial cell walls
- Forms a protective physical barrier when intact
- Move mucus and trapped particles out of the airways
The first line of defence consists of physical and chemical barriers that prevent pathogens from entering the body. These are non-specific, they work against all types of pathogens.
Skin: The largest organ of the body, covering about 2 square metres. The outer layer (epidermis) consists of dead, flattened cells filled with keratin, a tough protein that makes skin waterproof and resistant to pathogen penetration. Unless the skin is cut or damaged, most pathogens cannot penetrate it.
Mucous membranes: Line the respiratory, digestive, and urinary tracts. They secrete mucus, a sticky substance that traps pathogens, dust, and particles. The mucus is then moved along by cilia or swallowed into the stomach.
Cilia: Tiny hair-like projections on cells lining the respiratory tract. They beat in coordinated waves, sweeping mucus upward toward the throat where it can be swallowed or coughed out. This "mucociliary escalator" removes trapped pathogens before they can reach the lungs.
People who smoke damage their cilia, the toxic chemicals in cigarette smoke paralyse and destroy these delicate structures. Without functioning cilia, mucus and trapped pathogens accumulate in the airways, leading to chronic cough ("smoker cough") and increased susceptibility to respiratory infections like bronchitis and pneumonia. This is why smokers are much more likely to develop lung infections. When a smoker quits, cilia begin to regenerate within days to weeks, and the mucociliary escalator gradually recovers function. This recovery is one reason ex-smokers experience increased coughing initially, the cilia are working again to clear accumulated mucus.
Australian respiratory health: The Australian Lung Foundation reports that smoking remains the leading preventable cause of respiratory disease in Australia, despite declining smoking rates. Indigenous Australians have significantly higher smoking rates and correspondingly higher rates of respiratory infections and chronic lung disease. The Tackling Indigenous Smoking program, funded by the Australian Government, works with communities to reduce smoking and protect respiratory defences. Understanding how first-line barriers like cilia function, and how smoking damages them, is essential for designing effective public health interventions.
Tap each card to flip. Mark Got it when you can recall the answer without flipping.
In addition to physical barriers, the body deploys chemical barriers substances that kill or inhibit pathogens.
Stomach acid (hydrochloric acid): The stomach maintains a pH of 1.5-3.5, extremely acidic. Most bacteria that enter with food or drink are killed within minutes. This is why cholera (which survives stomach acid) and Helicobacter pylori (which burrows into the stomach lining) are notable exceptions.
Lysozyme: An enzyme found in tears, saliva, sweat, and breast milk. It breaks down peptidoglycan, a key component of bacterial cell walls. Without intact cell walls, bacteria burst and die. Lysozyme is particularly important for protecting the eyes, which are constantly exposed to airborne bacteria.
Sebum: An oily substance produced by sebaceous glands in the skin. It contains fatty acids and other compounds that inhibit bacterial and fungal growth, helping to prevent skin infections.
Defensins: Small antimicrobial peptides produced by skin and mucous membranes. They punch holes in bacterial membranes, killing them.
Breastfeeding provides newborns with passive immunity through antibodies (IgA) in breast milk, but it also delivers lysozyme, the enzyme that destroys bacterial cell walls. A single feed provides millions of units of lysozyme that help protect the infant immature gut from infection. This is one reason breastfed babies have lower rates of ear infections, diarrhoea, and respiratory infections than formula-fed babies. Australian health guidelines (NHMRC Infant Feeding Guidelines) recommend exclusive breastfeeding for the first 6 months because of these protective factors, among other benefits. The lysozyme in breast milk is a perfect example of how chemical barriers are adapted to specific life stages and vulnerabilities.
Australian antimicrobial peptide research: Researchers at the University of Melbourne and Monash University study defensins and other antimicrobial peptides produced by Australian wildlife. The platypus produces unique antimicrobial peptides in its milk (it lacks teats and secretes milk onto its skin). Tasmanian devils produce peptides that may help them survive facial tumour disease. These studies could lead to new antibiotics to combat drug-resistant bacteria. Australian researchers are also investigating how stomach acid levels affect susceptibility to gastrointestinal infections, with implications for proton pump inhibitor use, medications that reduce stomach acid and may increase infection risk.
5. A student compares how much harmless powder remains on hands after rinsing with water or washing with soap. In this fair test, the variable deliberately changed is the:
At the start of this lesson, you thought about how your skin, roughly 2 m² of living barrier, protects you from millions of pathogens every day, and what happens to a burn patient who loses that protection.
Now that you've studied the first line of defence, can you explain how your skin, mucus, and other physical barriers actually block pathogens? How does this change your appreciation of something as simple as an unbroken skin surface?
Q1. Describe two physical barriers and two chemical barriers of the first line of defence, and state how each one helps to stop pathogens.
Q2. Explain why the first line of defence is described as non-specific, and how it differs from the way the body later targets a particular pathogen.
Q3. A patient has lost a large area of skin to a burn. Explain how this affects their first line of defence and suggest two ways carers can help protect them from infection.
Model answers (click to reveal)
Answers
▾MCQ 1
A. Unbroken skin is a physical barrier that blocks pathogens from entering. Antibodies, memory T cells, and phagocytes belong to later, specific lines of defence.
MCQ 2
C. Lysozyme is a chemical defence that breaks down the cell walls of many bacteria, destroying them in tears, saliva, and other secretions.
MCQ 3
B. Cilia are tiny hair-like structures that beat to sweep mucus and trapped pathogens up and out of the airways.
MCQ 4
D. Stomach acid is a chemical barrier with a very low pH that kills many pathogens swallowed in food and drink before they can cause infection.
MCQ 5
B. A burn or deep cut breaks the skin, removing a key physical barrier and giving pathogens a direct route into the body.
Short Answer 1
Model answer: Physical barriers include the skin, which forms a tough, continuous covering that physically blocks pathogens, and the cilia and mucous membranes of the airways, where sticky mucus traps pathogens and cilia sweep them away. Chemical barriers include stomach acid, whose low pH kills many swallowed pathogens, and lysozyme in tears and saliva, an enzyme that breaks down bacterial cell walls. Sebum on the skin and the acidity of sweat are also acceptable chemical barriers. Each barrier stops pathogens before they can establish an infection.
Short Answer 2
Model answer: The first line of defence is non-specific because it acts in the same way against all pathogens, rather than recognising one particular type. Skin, mucus, cilia, stomach acid, and lysozyme block or destroy any pathogen they meet, regardless of what it is. This differs from later, specific defences, where the body recognises a particular pathogen by its antigens and produces tailored antibodies and memory cells against that exact pathogen. The first line is always ready and immediate, while the specific response is slower but targeted.
Short Answer 3
Model answer: Losing a large area of skin removes a major physical barrier of the first line of defence, so pathogens can enter the exposed tissue directly and the patient is at much higher risk of infection. The chemical protection of intact skin, such as sebum and acidic sweat, is also lost. Carers can help by (1) covering the wound with sterile dressings to replace the missing barrier and keep pathogens out, and (2) keeping the area and equipment clean and washing hands before contact to reduce the pathogens reaching the wound. Other valid answers include using a sterile environment, giving antibiotics if prescribed, and changing dressings carefully.