(2) How do our bodies defend themselves against infectious diseases?

Physical defence mechanisms of protection from pathogens

Your skin and hairs and mucous in the respiratory tract can stop a lot of the pathogen cells from entering your body.

When breathing, you cannot help taking in all sorts of fine particles of dust and microbes.

The whole of the respiratory tract from the nasal passage, down the trachea and into the lungs is covered with mucous and lined with ciliated cells. Cilia are fine hairs that can move freely at their ends.

The hairs and mucous in your nose traps dust and any other particles that might contain pathogens like bacteria, before they can get down  into the lungs.

The trachea and bronchi have ciliated epithelium - shown in the diagram below.

In between the ciliated epithelial cells are goblet cells that secrete mucus onto the surface of all the respiratory airways.

This sticky mucus traps particles like dust or microbe pathogens and the cilia (hairs) move the mucous along..

The hair-like structure of the cilia of the ciliated cells work together and move-push the mucous up to the back of the throat where it can be swallowed.

The ciliated cells have lots of mitochondria, they have a lot of work to do!

Cells that line the trachea and bronchi have cilia. These hair-like structures can move the mucous along from the lungs up to the nasal passage and back of the throat where it can be swallowed, coughed out or blow your nose, into a tissue!

Note that smoking can damage and paralyse the cilia reducing the ciliated cell's capacity to remove harmful particles, so another reason why smokers are more susceptible to respiratory diseases.

 

Skin in good condition acts as a very effective physical barrier against pathogens.

The outer layer of epidermis skin cells are dry and dead and pathogens cannot easily get through this layer.

The skin also protects the body from physical damage and dehydration.

As well as acting as a physical barrier, your skin also has sebaceous glands that secrete antimicrobial molecules that can kill pathogens.

The sebaceous glands are an 'offshoot' of the hair shaft, out of which the hair grows.

What happens if the skin is damaged?

When a cut in the skin occurs, small fragments of cells called platelets help the blood to clot quickly to seal the wound - the seal becomes the covering scab when dry, and prevent microorganisms entering the skin tissue or blood stream.

When the platelets are exposed to air through a cut, they become 'activated' and make protein fibres called fibrin, that form a mesh over the wound, and the mesh traps platelets and red blood cells to form a clot.

Clotting also reduces blood loss.

The greater the concentration of platelets in the blood the faster the clotting process ('sealing') can occur.

These physical defences are non-specific and can counteract a variety of types of pathogens.

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Key points Source of information is based on textbooks & syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10).

Key points about physical barriers that protect us against pathogens

A set of detailed revision notes focusing on the physical barriers of the human body and why they matter for understanding how we stay healthy:

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Physical Defences Against Pathogens

Our bodies are constantly exposed to pathogens like bacteria, viruses, and fungi. The first line of defence involves non-specific physical barriers that aim to prevent pathogens from entering the body in the first place.

1. Skin

• What it does: The skin acts as a tough, waterproof barrier that covers almost the entire body.

• How it works:

  • It's made of several layers, with the outermost layer (epidermis) consisting of dead cells.

  • These dead cells are continually shed, helping remove microbes that may be clinging to them.

  • The skin also secretes sebum, an oily substance that can kill or inhibit the growth of microbes.

2. Mucus and Cilia

• Found where: In the respiratory tract (nose, trachea, and bronchi).

• Function:

  • Mucus traps dust, pathogens, and other particles.

  • Ciliated epithelial cells sweep the mucus—along with the trapped pathogens—upwards toward the throat where it can be swallowed and destroyed in the stomach.

3. Tears (see section 3)

• Why they help: Produced by the lachrymal glands, tears constantly wash the surface of the eye.

• Defense mechanism: Contain lysozyme, an enzyme that breaks down bacterial cell walls.

4. Stomach Acid (see section 3)

• While not a physical barrier, it's important to note that swallowed pathogens (from food or mucus) are usually destroyed by hydrochloric acid in the stomach, providing further protection.

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Why This Topic Matters for Exams

• Foundation of immunity: Understanding physical defences is crucial before learning about more complex immune responses (like white blood cells and antibodies).

• Common exam question: Students are often asked to describe how the body defends itself non-specifically, so knowing these features thoroughly is key.

• Links to hygiene: These defences highlight why cuts should be cleaned, or why mucus should be cleared effectively—it’s about keeping that first barrier strong.

• Understanding disease spread: If these barriers are damaged (like broken skin), the body becomes more vulnerable—knowledge that ties into topics on vaccines, antibiotics, and public health.

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Learning objectives for this section on the physical ways the body defends itself against pathogens

Be able to describe physical ways in which the body defends and protects itself against pathogens.

Be able to describe how cilia hairs and mucous in the respiratory tract and the barrier of our skin help protects us against pathogens.

Be able to explain how the human body can be effective against attack from pathogens, including:

• The body has different physical and chemical ways of protecting itself against pathogens.

• Physical barriers – skin, cilia, mucus

  • Physical protection from pathogens

  • Your skin and hairs and mucous in the respiratory tract can stop a lot of the pathogen cells from entering your body. The whole of the respiratory tract from the nasal passage, down the trachea and into the lungs is covered with mucous and lined cilia (fine hairs that can move freely at their ends). The mucous traps dust and bacteria before they can get down into the lungs and the cilia move the mucous along from the lungs up to the nasal passage -and then you can blow your nose!

  • Skin in good condition acts as a very effective barrier against pathogens. When a cut in the skin occurs, small sections of cells called platelets help the blood to clot quickly to seal the wound (seal = scab when dry) and prevent microorganisms entering the skin tissue or blood stream. The greater the concentration of platelets in the blood the faster the clotting process ('sealing') can occur.

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