(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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