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If you've got a cold, 
mucus is hard to miss.

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But what is it, and what does it do
besides making you miserable?

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Your body produces more than a liter
of mucus every day,

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and all the wet surfaces of your body
that are not covered by skin,

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like your eyes,

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nose,

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mouth,

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lungs,

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and stomach get a liberal coating.

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That's why they're known 
as mucus membranes.

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Mucus plays lots of roles
in your body.

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It keeps delicate tissues 
from drying out and cracking,

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which would expose them to infection.

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It lubricates your eyes so you can blink.

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It protects your stomach lining
from acid.

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It neutralizes threats by removing
or trapping substances

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that could make you sick.

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And finally, it houses and keeps your
body's trillions of bacterial inhabitants,

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your microbiota, under control.

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Mucus contains lots 
of different compounds,

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including proteins, fats, and salts.

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But a key component of mucus versatility
is a set of proteins called mucins.

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Mucins are the primary 
large molecules in mucus

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and are essential 
for giving mucus its slippery feel.

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They belong to a class of proteins
called glycoproteins

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which are built out of both amino acids
and sugars.

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In mucin, long chains of sugars
are attached to specific amino acids

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in the protein backbone.

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The hydrophilic sugar chains help mucin
dissolve in your body's watery fluids.

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Mucus, which is up to 90% water,

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stays hydrated thanks 
to these sugar chains.

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Some of these mucins can interact
with other mucin molecules

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to create a complex network
that establishes a barrier

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against pathogens and other invaders.

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That's why mucus is the body's first line
of defense against foreign objects,

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like bacteria and dust.

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It's continuously produced to clear them
from the respiratory tract,

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like a slimy conveyor belt.

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This keeps bacteria from getting
a solid purchase on delicate lung tissue,

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or making it to the blood stream,
where they could cause a major infection.

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Many of those harmful bacteria
also cause diseases

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when they cluster into slimy growths
called biofilms.

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But mucus contains mucins,

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antimicrobial peptides,

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antibodies,

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and even bacteria-hungry viruses
called bacteriophages

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that all work together to prevent
biofilms from forming.

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If microbes do become harmful 
and you get sick,

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the body ramps up mucus production
to try to quickly flush out the offenders,

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and the immune system floods your mucus
with extra white blood cells.

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In fact, the greenish mucus often
associated with infections

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gets its color from an enzyme
produced by those white blood cells.

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This multi-pronged approach 
to bacterial management

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is one of the main reasons 
why we're not sick all the time.

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Even though mucus protects against
the infectious bacteria,

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the vast majority of your body's bacterial
tenants are not harmful,

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and many are actually beneficial.

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That's particularly true 
when they live in mucus,

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where they can perform 
important functions,

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like synthesizing vitamins,

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suppressing harmful inflammation,

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and controlling the growth 
of more harmful species.

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So even though you probably associate
mucus with being ill,

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it's really helping you stay healthy.

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Sure, it might seem gross,

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but can you think of any other substance
that can lubricate,

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keep your body clean,

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fight infection,

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and domesticate a teeming 
bacterial population?

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Nope, just mucus.