Physical and Chemical Barriers

The human body has a range of non-specific defences that act as the first line of defence against pathogens. These defences work against all pathogens — they do not target specific ones. In this lesson you will learn about the physical and chemical barriers that prevent pathogens from entering the body.

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Overview: Lines of Defence

The body's defences can be thought of in layers:

Line of defence	Type	How it works
First line	Non-specific (physical and chemical barriers)	Prevents pathogens from entering the body
Second line	Non-specific (immune cells)	Phagocytes engulf and destroy pathogens that get past the first line
Third line	Specific (immune response)	Lymphocytes produce antibodies and antitoxins targeted at specific pathogens

This lesson focuses on the first line of defence — the physical and chemical barriers.

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Physical Barriers

1. The Skin

• The skin is the largest organ of the body and acts as a continuous physical barrier
• The outer layer of skin (epidermis) is made of tough, dead cells that are difficult for pathogens to penetrate
• Skin cells are constantly shed and replaced, removing any pathogens attached to the surface
• Sebaceous glands in the skin produce sebum (an oily substance) that has antimicrobial properties

When the skin is damaged (cuts and wounds):

• Pathogens can enter through breaks in the skin
• This is why wound healing is so important as a defence mechanism (see below)

2. Nose Hairs and Mucus

• The nose is a major entry point for airborne pathogens
• Nose hairs act as a physical filter, trapping large particles, dust, and some pathogens
• The nasal passages are lined with mucous membranes that produce sticky mucus
• Mucus traps smaller particles and pathogens, preventing them from reaching the lungs

3. The Respiratory Tract — Ciliated Epithelial Cells and Goblet Cells

This is one of the most important non-specific defence mechanisms, and it is frequently tested.

The trachea and bronchi are lined with two key cell types:

Cell type	Function
Goblet cells	Produce mucus — a sticky substance that traps pathogens, dust, and other particles
Ciliated epithelial cells	Have tiny hair-like projections called cilia that beat in a rhythmic wave motion, sweeping the mucus (and trapped pathogens) upwards towards the throat

The process:

1. Pathogens are breathed in and get trapped in mucus produced by goblet cells
2. Cilia on the ciliated epithelial cells beat rhythmically, moving the mucus upwards towards the throat
3. When the mucus reaches the throat, it is swallowed into the stomach
4. Stomach acid destroys the trapped pathogens

This is sometimes called the mucociliary escalator.

Why smoking damages this defence:

• Tobacco smoke paralyses and destroys cilia, so mucus (and trapped pathogens) is not moved out of the airways
• This leads to mucus building up in the lungs, causing the smoker's cough as the body tries to remove it
• The damaged airways are more vulnerable to infection, which is why smokers are more prone to respiratory infections such as bronchitis and pneumonia

Exam tip: Make sure you know the difference between goblet cells (produce mucus) and ciliated epithelial cells (have cilia that move mucus). A very common exam mistake is to say that cilia trap pathogens — they do not. The mucus traps pathogens; the cilia move the mucus.

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Chemical Barriers

1. Stomach Acid (Hydrochloric Acid)

• The stomach produces hydrochloric acid (HCl)
• This creates a highly acidic environment with a pH of approximately 1–2
• Most pathogens cannot survive in such acidic conditions
• This is why swallowing mucus from the respiratory tract is an effective defence — the pathogens trapped in the mucus are destroyed by stomach acid
• Stomach acid also kills most pathogens present in food and drink

2. Lysozyme Enzyme

• Lysozyme is an enzyme found in several body fluids:

  • Tears (lacrimal fluid)
  • Saliva
  • Nasal secretions
  • Breast milk

• Lysozyme works by breaking down (hydrolysing) the cell walls of bacteria

• Specifically, it breaks the bonds in peptidoglycan, the main structural component of bacterial cell walls

• When the cell wall is damaged, the bacterium bursts (lysis) due to osmotic pressure — water enters the cell and it cannot maintain its shape

Exam tip: Lysozyme in tears and saliva is a chemical defence, not a physical one. It works by destroying bacterial cell walls through enzymatic action. This is a good example of a non-specific defence because it works against many different types of bacteria.

3. Sebum and Skin Secretions

• Sebum is an oily substance produced by sebaceous glands in the skin
• It has antimicrobial properties — it creates a slightly acidic environment on the skin surface (pH ~5.5) that inhibits the growth of many pathogens
• Sweat also contains antimicrobial substances, including lysozyme and dermcidin

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Wound Healing and Blood Clotting

When the skin is broken, the body must seal the wound quickly to prevent pathogens from entering. This is achieved through blood clotting.

The Blood Clotting Process

1. When a blood vessel is damaged, platelets (cell fragments in the blood) are activated
2. Platelets gather at the wound site and form a platelet plug
3. Platelets and damaged tissue release chemical signals that trigger the clotting cascade — a series of enzyme reactions
4. The protein fibrinogen (dissolved in plasma) is converted to fibrin
5. Fibrin forms a mesh of fibres across the wound
6. Red blood cells and platelets get trapped in the fibrin mesh, forming a blood clot
7. The clot dries to form a scab, which protects the wound while new skin cells grow underneath

Why Blood Clotting Is a Defence