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.