Body defences

Diseases and immunity · Pathogens and transmission · note 2 of 4

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Body defencesSpec 10.1.4

In short

The body's defences against pathogens include barriers that keep them out and cells that destroy those that get in. The skin is a tough barrier, nose hairs and mucus trap pathogens, and stomach acid kills most swallowed pathogens. White blood cells destroy pathogens inside the body by ingesting them or by making antibodies.

The body has defences that stop pathogens getting in, and others that destroy the ones that do get in.

Body defences
Defence	How it protects the body
Skin	Forms a tough barrier that stops pathogens entering the body.
Hairs in the nose	Trap dust and pathogens in the air that is breathed in.
Mucus	Sticky liquid in the nose and airways that traps pathogens so they can be removed.
Stomach acid	Kills most pathogens that are swallowed in food and drink.
White blood cells	Destroy pathogens that get inside the body, for example by ingesting them (phagocytes) or by making antibodies (lymphocytes).

Common mistake:

Hairs and mucus do not kill pathogens. They trap them. Stomach acid and white blood cells are the defences that kill or destroy pathogens.

Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026

Frequently asked questions

How do pathogens spread?

Pathogens spread by direct contact or indirectly. Direct contact includes blood and other body fluids passing from one person to another, for example through a cut. Indirect routes are contaminated surfaces such as door handles, contaminated food and water, animals such as mosquitoes, and tiny droplets in the air from coughs and sneezes.

How does the body stop pathogens getting in?

The skin forms a tough barrier that stops pathogens entering the body. Hairs in the nose and sticky mucus in the airways trap pathogens so they can be removed, and stomach acid kills most pathogens swallowed in food and drink. White blood cells then destroy any pathogens that do get inside the body.

Why is clean water important for preventing disease?

Clean water is important because drinking water can carry pathogens. Treating the water supply removes them and stops water-borne diseases such as cholera from spreading. Sewage treatment also matters, because sewage contains pathogens from human faeces, and treating it stops them contaminating drinking water or food crops.

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