Human Defence Systems — Non-Specific

The human body has several lines of defence that work together to prevent pathogens from entering and causing disease. This lesson focuses on the non-specific defences — barriers and mechanisms that protect against all pathogens, regardless of type. This is a key topic in the AQA GCSE Combined Science Trilogy specification (8464).

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Specific vs Non-Specific Defence

Before studying the defences in detail, it is important to understand the difference:

Type	Description	Examples
Non-specific defence	Works against all pathogens equally; does not target a specific pathogen	Skin, mucus, stomach acid, cilia
Specific defence	Targets a particular pathogen using the immune system	White blood cells producing antibodies

Non-specific defences are sometimes called the body's first line of defence because they act before the immune system is activated.

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The Body's Non-Specific Defences

1. The Skin

The skin is the body's largest organ and acts as a continuous physical barrier to prevent pathogens entering:

Feature	How It Protects
Intact skin	Forms a waterproof, continuous barrier that pathogens cannot penetrate
Cuts and wounds	When skin is broken, blood clots form quickly to seal the wound and prevent pathogen entry
Antimicrobial secretions	The skin produces antimicrobial substances and oils that inhibit bacterial growth
Dead skin cells	The outer layer (epidermis) is made of dead cells that are constantly shed, removing pathogens on the surface

Exam Tip: If asked how the body prevents pathogen entry, always mention the skin first — it is the most obvious and important physical barrier.

2. The Nose — Hairs and Mucus

Defence	How It Works
Nasal hairs	Trap larger particles and some pathogens before they can enter the airways
Mucus	Sticky substance that lines the nasal passages and airways; traps smaller particles and pathogens

3. The Trachea and Bronchi — Cilia and Mucus

The respiratory tract has a specialised defence system:

Structure	Function
Goblet cells	Secrete mucus that traps pathogens and dust particles in the airways
Ciliated epithelial cells	Have tiny hair-like projections called cilia that beat rhythmically to sweep mucus upwards towards the throat
Throat	Mucus (containing trapped pathogens) is swallowed into the stomach

graph LR
    A[Pathogens inhaled] --> B[Trapped in mucus in airways]
    B --> C[Cilia sweep mucus upwards]
    C --> D[Mucus reaches throat]
    D --> E[Swallowed into stomach]
    E --> F[Destroyed by stomach acid]

Exam Tip: Cilia sweep mucus up to the throat, not down to the lungs. This is a common exam mistake. The mucus is then swallowed and the pathogens are destroyed by stomach acid.

4. Stomach Acid

Feature	Detail
Substance	Hydrochloric acid (HCl)
pH	Approximately pH 2 (strongly acidic)
Function	Kills the majority of pathogens that are swallowed with food, drink, or mucus from the airways
How it kills pathogens	The low pH denatures the enzymes inside the pathogen, preventing them from carrying out the chemical reactions needed for survival

Exam Tip: The word "denatures" is important here. Stomach acid denatures the enzymes and other proteins in pathogens, destroying their function. Do not say it "dissolves" them.

5. Tears and Saliva — Lysozyme

Feature	Detail
Substance	The enzyme lysozyme
Found in	Tears, saliva, and nasal secretions
Function	Breaks down the cell walls of bacteria, causing them to burst (lysis)

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Summary Diagram — The Body's Non-Specific Defences

graph TD
    A[Non-Specific Defences] --> B[Physical Barriers]
    A --> C[Chemical Barriers]
    B --> D[Skin — continuous barrier]
    B --> E[Nose hairs and mucus — trap particles]
    B --> F[Cilia in airways — sweep mucus to throat]
    C --> G[Stomach acid — pH 2, kills pathogens]
    C --> H[Lysozyme in tears/saliva — destroys bacterial cell walls]
    C --> I[Antimicrobial skin secretions]

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How These Defences Work Together

The non-specific defences form a layered system:

Line of Defence	Structures Involved	Action
External barriers	Skin, nasal hairs	Physically block pathogen entry
Airway defences	Mucus, cilia, goblet cells	Trap and remove pathogens from respiratory tract
Chemical defences	Stomach acid, lysozyme	Chemically destroy pathogens that bypass physical barriers

If pathogens manage to get past all of these non-specific defences (for example, through a wound or by entering the bloodstream), then the body's immune system (specific defence) takes over — this is covered in the next lesson.

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Practical Connection — The Effect of Antiseptics

You may have carried out a practical investigating the effectiveness of antiseptics or disinfectants on bacterial growth:

Step	Detail
1	Prepare agar plates with a lawn of bacteria
2	Place paper discs soaked in different antiseptics on the agar
3	Incubate at no more than 25 °C (in schools, to prevent growth of human pathogens)
4	Measure the zone of inhibition — the clear area around each disc where bacteria have not grown
5	A larger zone = more effective antiseptic

The area of the zone of inhibition can be calculated using:

$A = \pi r^2$A=πr2

Where $r$r is the radius of the clear zone (measured from the edge of the disc).

Exam Tip: You may be asked to calculate the area of a zone of inhibition. Always measure the radius, not the diameter, and use $\pi r^2$πr2. Show your working and include units ($\text{mm}^2$mm2).

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Common Mistakes

Mistake	Correction
"Cilia push pathogens down to the lungs"	Cilia sweep mucus upwards to the throat
"Stomach acid dissolves pathogens"	Stomach acid denatures enzymes in pathogens
"The skin produces antibodies"	The skin is a physical barrier — antibodies are produced by white blood cells
"Non-specific means it only works on one pathogen"	Non-specific means it works against all pathogens equally

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Summary