Year 9 Science Unit 1 · Disease Lesson 5 of 20 45 min SC5-DIS-05

First Line of Defence

Your body is under constant attack. Every moment, billions of microorganisms try to invade. Yet most never make it past your outer defences. Your skin, tears, saliva, and stomach acid form an invisible shield that stops pathogens before they can cause harm.

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Think First

Before You Begin

Think about a time you got a small cut or grazed your knee. The skin was broken, and maybe it got a bit red or swollen.

Write down your answers before reading on:

  • Why does the skin normally keep you safe from infection?
  • What happens when that barrier is broken?
  • What other parts of your body might also stop germs from getting in?
Write your thinking in your book before reading on.

Work mode: Digital — answers typed below

Learning Intentions

Know

  • The components of the first line of defence: skin, mucous membranes, stomach acid, tears, saliva
  • How each barrier works to prevent pathogen entry
  • That the first line of defence is non-specific

Understand

  • Why the first line of defence is called "non-specific"
  • How physical barriers differ from chemical barriers
  • What happens when first-line defences are compromised

Can Do

  • Identify the first-line defence mechanisms in different body systems
  • Explain how each barrier prevents pathogen entry
  • Describe the consequences when first-line defences fail
Key Terms
First line of defence The body's initial non-specific barriers that prevent pathogens from entering: skin, mucous membranes, and chemical secretions.
Physical barrier A structural obstacle that physically blocks pathogens from entering the body, such as skin or mucous membranes.
Chemical barrier A substance that kills or inhibits pathogens, such as stomach acid, enzymes in tears, or antimicrobial proteins in saliva.
Mucous membrane A moist tissue layer lining body openings and internal passages that traps pathogens in sticky mucus.
Stomach acid Hydrochloric acid in the stomach that kills most ingested pathogens.
Non-specific defence An immune response that acts against all pathogens in the same way, regardless of type.
Core Content
1

Skin — The Body's Armour

Your largest organ is your first shield

Transmission Routes

Transmission Routes

Your skin is the largest organ of your body and your most important physical barrier. It covers about 2 square metres and acts as a tough, waterproof shield.

Skin prevents pathogen entry in several ways:

  • Physical barrier: The outer layer (epidermis) is made of dead, flattened cells that pathogens cannot easily penetrate.
  • Constant shedding: Skin cells are replaced every 2-4 weeks, carrying away attached microbes.
  • Low pH: Sweat and skin oils create an acidic environment (pH ~5.5) that inhibits bacterial growth.
  • Beneficial bacteria: Normal skin flora compete with and suppress harmful pathogens.

When skin is broken by cuts, grazes, or burns, pathogens can enter. This is why keeping wounds clean and covered is so important — you are restoring your first line of defence.

2

Mucous Membranes

Trapping invaders at every opening

Mucous membranes line all body openings and internal passages that connect to the outside world: the respiratory tract, digestive tract, urinary tract, and reproductive tract.

They defend against pathogens by:

  • Trapping pathogens: Mucus is a sticky substance that captures microbes before they can enter tissues.
  • Cilia movement: In the respiratory tract, tiny hair-like structures called cilia sweep mucus (with trapped pathogens) up and out of the lungs.
  • Secreting antimicrobial substances: Mucus contains enzymes and antibodies that kill or neutralise pathogens.

When you have a cold, your body produces extra mucus to trap viruses — this is why you get a runny nose. Coughing and sneezing are also defence mechanisms that expel trapped pathogens.

3

Chemical Barriers

Acid and enzymes that kill

Beyond physical barriers, your body produces chemical substances that kill or inhibit pathogens:

Stomach acid (hydrochloric acid): Your stomach maintains a highly acidic environment (pH 1.5-3.5) that kills most ingested bacteria and viruses. This is one reason you can eat food containing small numbers of pathogens without getting sick.

Tears: Contain lysozyme, an enzyme that breaks down bacterial cell walls. This protects your eyes from infection.

Saliva: Also contains lysozyme and other antimicrobial compounds that reduce bacterial growth in the mouth.

Earwax: Traps dust and microbes, preventing them from reaching the eardrum.

Urine: Its acidic pH and flushing action help prevent urinary tract infections.

4

When First-Line Defences Fail

What happens when the shield breaks

First-line defences are remarkably effective, but they can fail:

  • Skin breaks: Cuts, burns, and surgical wounds allow direct pathogen entry. Staphylococcus aureus commonly enters through skin breaks.
  • Mucous membrane damage: Smoking damages cilia in the respiratory tract, making it easier for pathogens to reach the lungs.
  • Reduced stomach acid: Some medications reduce stomach acid, increasing susceptibility to foodborne illness.
  • Overwhelming numbers: Exposure to very high numbers of pathogens can overwhelm defences.

When first-line defences are breached, the second line of defence (inflammation, phagocytes, fever) activates. These defences are still non-specific but act inside the body to contain and eliminate invaders.

Common Misconceptions

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"Sweat causes disease by letting germs grow." No — sweat actually helps protect against infection. Its acidic pH and antimicrobial peptides inhibit bacterial growth on the skin.

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"Mucus is just a nuisance when you have a cold." No — mucus is a crucial defence mechanism. It traps pathogens so they can be expelled. A runny nose is your body working to protect you.

✗trong>"If you swallow germs, you will always get sick." No — stomach acid kills most ingested pathogens. You would need to swallow very large numbers of pathogens, or have reduced stomach acid, to become ill.

Australian Context

Australian Skin Infections

Golden staph (MRSA) in Australia: Staphylococcus aureus — commonly called golden staph — lives harmlessly on the skin of about 30% of Australians. However, when skin is broken, it can cause serious infections. Methicillin-resistant S. aureus (MRSA) is a strain resistant to many antibiotics. Australian hospitals have strict infection control protocols to prevent MRSA spread, including hand hygiene programs and patient screening.

Skin infections in Indigenous communities: Aboriginal and Torres Strait Islander children in remote communities experience very high rates of skin infections, including scabies and streptococcal skin sores. These infections enter through damaged skin and are linked to overcrowded housing and limited access to healthcare. The Healthy Skin Program works with communities to reduce these infections through hygiene education and treatment.

Trachoma elimination: Trachoma is a bacterial eye infection spread by contact with eye and nose discharges. It is the world's leading infectious cause of blindness. Australia is the only developed country where trachoma still occurs, mainly in remote Aboriginal communities. Through the SAFE strategy (Surgery, Antibiotics, Facial cleanliness, Environment), Australia is working toward eliminating trachoma.

✍ Copy Into Your Books

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Physical Barriers

  • Skin: tough, waterproof, acidic, sheds regularly
  • Mucous membranes: trap pathogens in sticky mucus
  • Cilia: sweep mucus out of respiratory tract

Chemical Barriers

  • Stomach acid (HCl): pH 1.5-3.5 kills ingested pathogens
  • Tears: contain lysozyme that breaks bacterial cell walls
  • Saliva: contains lysozyme and antimicrobial compounds

Key Concept

  • First line of defence = non-specific
  • Acts against ALL pathogens the same way
  • Prevents entry rather than fighting infection
Activities
Activity 1

Barrier Identification

Match body parts to their defensive functions.

1 Explain how each of the following acts as a first-line defence: skin, mucous membranes in the nose, stomach acid, tears.
Answer in your book.
2 A person taking medication that reduces stomach acid is more likely to get food poisoning. Explain why.
Answer in your book.
3 Smokers often get more chest infections than non-smokers. Use your knowledge of first-line defences to explain why.
Answer in your book.
Activity 2

Design an Experiment

Test the effect of a first-line defence.

1 Design an experiment to test whether skin bacteria inhibit the growth of other bacteria. Include your hypothesis, method, and variables.
Answer in your book.
2 Predict what would happen to infection rates if everyone in a population suddenly lost their skin. Explain your reasoning.
Answer in your book.
3 Research and describe one way that modern medicine helps restore first-line defences when they are damaged.
Answer in your book.
Multiple Choice
Q

Test Your Understanding

RememberBand 3

1. Which of the following is a physical barrier in the first line of defence?

AStomach acid
BLysozyme in tears
CSkin
DAntibodies
UnderstandBand 3

2. What is the function of cilia in the respiratory tract?

ATo produce mucus
BTo sweep mucus and trapped pathogens out of the lungs
CTo absorb oxygen
DTo produce antibodies
RememberBand 3

3. Stomach acid kills most ingested pathogens because it is:

AVery cold
BHighly acidic
CHighly alkaline
DRich in sugar
UnderstandBand 4

4. Why is the first line of defence described as "non-specific"?

AIt only targets bacteria
BIt acts against all pathogens in the same way
CIt does not work very well
DIt only protects the skin
ApplyBand 3

5. A cut on your finger becomes infected. Which first-line defence has been compromised?

AAntibodies
BSkin
CFever
DWhite blood cells
Short Answer
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Short Answer Questions

RememberBand 3

1. Describe the three main types of first-line defence (physical barriers, chemical barriers, and mechanical processes) and give one example of each. 4 MARKS

Answer in your book.
ApplyBand 4

2. Explain why a person with severe burns is at high risk of infection, using your knowledge of first-line defences. 4 MARKS

Answer in your book.
AnalyseBand 5

3. Compare the first line of defence with the immune system (third line). How do they differ in specificity, speed, and mechanism? 4 MARKS

Answer in your book.

Revisit Your Thinking

Go back to your Think First answer. Has your understanding changed?

Update your thinking in your book.

Answers

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MCQ 1

C — Skin is a physical barrier that physically blocks pathogens from entering the body. Stomach acid and lysozyme are chemical barriers. Antibodies are part of the third line of defence.

MCQ 2

B — Cilia are tiny hair-like structures that beat in coordinated waves to sweep mucus (with trapped pathogens) up and out of the respiratory tract.

MCQ 3

B — Stomach acid (hydrochloric acid) creates a highly acidic environment with pH 1.5-3.5, which kills most bacteria and viruses that are ingested with food.

MCQ 4

B — The first line of defence is non-specific because it acts against all types of pathogens in the same way, regardless of what the pathogen is. It does not target specific pathogens.

MCQ 5

B — A cut compromises the skin, which is the primary physical barrier of the first line of defence. This allows pathogens to enter and cause infection.

Short Answer 1

Model answer: The three main types of first-line defence are: (1) Physical barriers — structural obstacles that block pathogen entry. The skin is the most important example, acting as a tough, waterproof shield. Mucous membranes are another physical barrier. (2) Chemical barriers — substances that kill or inhibit pathogens. Stomach acid (hydrochloric acid) kills most ingested microbes. Tears contain lysozyme, an enzyme that breaks down bacterial cell walls. Saliva also contains antimicrobial compounds. (3) Mechanical processes — physical actions that remove pathogens. Cilia in the respiratory tract sweep mucus (with trapped microbes) out of the lungs. Coughing, sneezing, and urination also mechanically expel pathogens.

Short Answer 2

Model answer: A person with severe burns is at high risk of infection because the skin — the body's largest and most important physical barrier — has been destroyed over a large area. Without intact skin, pathogens can enter directly into underlying tissues and the bloodstream. The skin's protective acidic environment and beneficial bacteria are also lost. Additionally, burned tissue provides a moist, nutrient-rich environment where bacteria can multiply rapidly. This is why burn patients are treated in sterile environments and receive antibiotics preventively.

Short Answer 3

Model answer: The first line of defence and the immune system (third line) differ in several key ways. Specificity: The first line is non-specific — it acts against all pathogens in the same way, regardless of type. The third line is specific — lymphocytes produce antibodies tailored to particular pathogens. Speed: The first line acts immediately and constantly. The third line takes days to mount a full response because it must first identify the specific pathogen. Mechanism: The first line uses physical barriers (skin, mucous membranes) and chemical substances (stomach acid, lysozyme) to prevent entry. The third line uses cellular responses — B cells produce antibodies, T cells kill infected cells, and memory cells enable faster future responses. The first line prevents infection; the third line fights infection that has already entered.

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Lesson Game

Barrier Blitz

Defend the body! Position physical and chemical barriers to stop waves of pathogens from breaching the body's defences.

Mark lesson as complete

Tick when you have finished all activities and checked your answers.

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