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External Defences

Human Biology (Year 12) - Defence Against Pathogens

Ben Whitten

What are the body's external defences?

The body has a number of external defences which actively protects it from any invading pathogens, including the skin, sebum, sweat, mucous membranes, saliva, acid secretions, the urethra, mucous, cilia and hairs, cerumen and a flushing action.


The skin provides the body with a waterproof barrier. This prevents pathogens from penetrating and entering the internal body system. Alongside this, bacterial colonies found on the surface of the skin make it difficult for pathogens to penetrate and enter the body.


Sebum is an oily secretion which is slightly acidic, and its acidity creates an environment unsuitable for a majority of pathogens.


Sweat is a secretion of water, salts, wastes and fatty acids from the skin. It is these salts and fatty acids which prevents the growth of pathogens from occurring.

Mucous membranes:

Mucous is secreted onto the inner lining of the digestive tract, and this actively prevents bacteria and other pathogens from entering the different organs found in the body.


Lysozyme is an enzyme found in saliva which actively kills bacteria; it creates a flushing/cleansisng action which eliminates any invading bacteria.

Acid secretions:

Acidity kills bacteria and reduces their growth. Acid secretions are found in the digestive tract, for example the stomach has a pH level of 2, as well as in the vagina.


The urethra prevents a build up of pathogens from occurring through a flushing/cleanising action (urinating) which actively eliminates any pathogens.

Cilia and hairs:

Cilia/hairs are tiny finger-like projections (similar to villi found in the small intestine) which trap microoorganisms, and they move pathogens out of the respiratory tract with wave-like contractions.


Another name for ear wax, is slightly acidic and contains lysozyme; it provides defence for the body as it breaks down bacteria/pathogens and prevents entry.

Flushing action:

Contains lysozyme, and tears prevent bacteria from having an environment to grow.

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