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

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

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

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

like your eyes,

nose,

mouth,

lungs,

and stomach get a liberal coating.

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

Mucus plays lots of roles
in your body.

It keeps delicate tissues 
from drying out and cracking,

which would expose them to infection.

It lubricates your eyes so you can blink.

It protects your stomach lining
from acid.

It neutralizes threats by removing
or trapping substances

that could make you sick.

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

your microbiota, under control.

Mucus contains lots 
of different compounds,

including proteins, fats, and salts.

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

Mucins are the primary 
large molecules in mucus

and are essential 
for giving mucus its slippery feel.

They belong to a class of proteins
called glycoproteins

which are built out of both amino acids
and sugars.

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

in the protein backbone.

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

Mucus, which is up to 90% water,

stays hydrated thanks 
to these sugar chains.

Some of these mucins can interact
with other mucin molecules

to create a complex network
that establishes a barrier

against pathogens and other invaders.

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

like bacteria and dust.

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

like a slimy conveyor belt.

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

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

Many of those harmful bacteria
also cause diseases

when they cluster into slimy growths
called biofilms.

But mucus contains mucins,

antimicrobial peptides,

antibodies,

and even bacteria-hungry viruses
called bacteriophages

that all work together to prevent
biofilms from forming.

If microbes do become harmful 
and you get sick,

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

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

In fact, the greenish mucus often
associated with infections

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

This multi-pronged approach 
to bacterial management

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

Even though mucus protects against
the infectious bacteria,

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

and many are actually beneficial.

That's particularly true 
when they live in mucus,

where they can perform 
important functions,

like synthesizing vitamins,

suppressing harmful inflammation,

and controlling the growth 
of more harmful species.

So even though you probably associate
mucus with being ill,

it's really helping you stay healthy.

Sure, it might seem gross,

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

keep your body clean,

fight infection,

and domesticate a teeming 
bacterial population?

Nope, just mucus.