Ways in

Barriers help protect our insides from outside

One organ acts as your largest and perhaps most important barrier against infection. Spread out on the ground, it would cover two square metres: your skin. It’s tough – skin cells are packed full of strong structural proteins called keratins, which act as a barrier. As the cells grow outwards, they die. The stratum corneum – the outermost layer of the epidermis, which is the outermost layer of the skin – is completely dead. Viruses can’t replicate in dead cells. Microbes are often outcompeted by bacteria that live on our skin in a mutualistic relationship with us.

Inside our bodies, sticky mucous membranes line our airways and guts, as well as the tracts in our urinary and reproductive systems. These need to be protected because they are constantly exposed to microbes – for example, in the air we breathe and the food we eat. Mucus traps and destroys microbes by deploying enzymes such as lysozyme, which breaks down bacterial cell walls. Cells in the stomach lining secrete hydrochloric acid, creating an acidic environment too inhospitable for many bacteria.

However some bacteria, such as Helicobacter pylori, which cause stomach ulcers, have adapted their defences. These bacteria produce an enzyme called urease, which uses urea from human tissues to make ammonia – an alkaline chemical that neutralises the acid.

If something goes wrong with these barriers, we become more susceptible to disease. For instance, some people have genetic mutations that affect a protein in the skin called filaggrin. Filaggrin plays an important role in strengthening the skin, by helping organise the keratin filaments in skin cells and also by moisturising the skin. Filaggrin mutations weaken the skin barrier, allowing allergens to enter through it. This leads to some cases of eczema, asthma and hay fever.

Lead image:

dann toliver/Flickr CC BY NC


About this resource

This resource was first published in ‘Immune System’ in January 2015.

Microbiology, Immunology, Health, infection and disease
Immune System
Education levels:
14–16, 16–19, Continuing professional development