Human Defence Systems

This lesson covers the body's non-specific defence systems as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the physical and chemical barriers that form the first line of defence against pathogens, including the skin, mucous membranes, cilia, stomach acid and other mechanisms.

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Lines of Defence

The human body has a multi-layered defence system to prevent pathogens from entering and causing disease. These defences are divided into two main categories:

1. Non-specific defences (first line of defence) — physical and chemical barriers that protect against ALL pathogens. They do not target specific pathogens.
2. Specific defences (the immune response) — targeted responses by white blood cells against particular pathogens. This is covered in the next lesson.

graph TD
    A["Pathogen attempts<br/>to enter body"] --> B{"First line of defence<br/>Non-specific barriers"}
    B -->|Pathogen blocked| C[No infection]
    B -->|Pathogen gets through| D{"Second line of defence<br/>Immune response"}
    D -->|Pathogen destroyed| E[Recovery]
    D -->|Pathogen not controlled| F[Disease develops]

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The Skin

The skin is the body's largest organ and the most important physical barrier against infection.

How the Skin Protects

Feature	How it prevents infection
Continuous physical barrier	The outer layer (epidermis) is made of tightly packed, tough cells that pathogens cannot easily penetrate
Dead outer cells	The outermost layer consists of dead cells filled with a tough protein called keratin, making it waterproof and resistant to pathogens
Constantly replaced	Skin cells are shed and replaced regularly, removing pathogens that have attached to the surface
Antimicrobial secretions	Skin glands produce sebum (an oily substance) that has antimicrobial properties

What Happens When Skin Is Broken?

If the skin is cut or wounded, pathogens can enter the body through the break. The body responds with blood clotting:

1. Platelets (cell fragments in the blood) arrive at the wound site.
2. Platelets release chemicals that trigger a cascade of reactions.
3. A protein called fibrinogen is converted to fibrin.
4. Fibrin forms a mesh of fibres across the wound.
5. Red blood cells become trapped in the fibrin mesh, forming a clot (scab).
6. The scab seals the wound, preventing pathogens from entering while the skin heals beneath.

graph LR
    A["Skin cut<br/>or wound"] --> B["Platelets arrive<br/>at wound site"]
    B --> C["Chemical signals<br/>trigger clotting cascade"]
    C --> D["Fibrinogen converted<br/>to fibrin"]
    D --> E["Fibrin mesh traps<br/>red blood cells"]
    E --> F["Clot / scab forms<br/>sealing the wound"]

Exam Tip: The clotting process is a common exam question. Remember the key steps: platelets arrive, fibrinogen is converted to fibrin, fibrin forms a mesh, and the clot/scab seals the wound.

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The Respiratory System Defences

Pathogens in the air can enter the body through the nose and mouth when we breathe. The respiratory system has several defences:

Mucous Membranes

Mucous membranes line the airways (nose, trachea, bronchi). They contain goblet cells that secrete mucus — a thick, sticky fluid.

• Mucus traps pathogens, dust and other particles before they can reach the lungs.
• Mucus also contains lysozyme, an enzyme that breaks down bacterial cell walls.

Cilia

Cilia are tiny, hair-like structures on the surface of epithelial cells lining the airways.

• Cilia beat in a coordinated, wave-like motion.
• They sweep mucus (containing trapped pathogens) upwards towards the throat.
• The mucus is then swallowed into the stomach, where acid destroys the pathogens.

Structure	Location	Function
Goblet cells	Lining of airways	Secrete mucus
Mucus	Coating the airways	Traps pathogens and particles
Cilia	Surface of airway epithelial cells	Sweep mucus towards the throat

Exam Tip: Smoking damages and paralyses cilia. This is why smokers are more prone to chest infections — mucus (and trapped pathogens) is not swept away efficiently, so it accumulates in the lungs.

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Stomach Acid

The stomach produces hydrochloric acid (HCl), creating a very acidic environment (approximately pH 2).

How Stomach Acid Protects

• Stomach acid kills most pathogens that are swallowed — either in food, in water, or in mucus that has been swept down from the airways.
• The low pH denatures the enzymes of pathogens, destroying them.
• This is an important chemical barrier against infection.

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Other Non-specific Defences

Defence	Location	How it protects
Tears	Eyes	Contain lysozyme, which destroys bacteria by breaking down their cell walls
Nasal hairs	Nose	Trap larger particles and some pathogens before they enter the airways
Earwax (cerumen)	Ear canal	Traps dust and pathogens; contains antimicrobial substances
Urine flow	Urinary tract	The flow of urine flushes pathogens out of the urethra
Normal flora	Skin, gut, other surfaces	Beneficial bacteria that compete with pathogens for space and nutrients

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Summary Table of Non-specific Defences

Defence	Type	Mechanism
Skin	Physical / chemical	Tough barrier; keratin; antimicrobial sebum
Blood clotting	Physical	Fibrin mesh seals wounds
Mucus	Physical / chemical	Traps pathogens; contains lysozyme
Cilia	Physical	Sweep mucus to throat for swallowing
Stomach acid	Chemical	Kills pathogens at pH 2; denatures enzymes
Tears	Chemical	Lysozyme destroys bacterial cell walls
Nasal hairs	Physical	Trap particles and pathogens

Exam Tip: In a question about non-specific defences, describe both the structure AND how it prevents infection. For example, do not just say "skin protects against pathogens" — explain that the skin is a continuous physical barrier of tightly packed cells containing keratin.

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Summary