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Dear Science: What’s the point of mucus?

(Washington Post illustration; iStock)

Dear Science,

It's that time of year. My nose is stuffed up, and I can't pronounce the letter “n,” and I have to know — snuffle — what's the deal with mucus? What is it? Why is it so slimy? Why is there so much of it? What's the — sniff — point?

Here's what science has to say:

Our sympathies for your cold. We haven't been doing too great ourselves — achoo!

But we urge you to pause for a moment before blaming mucus for your troubles. That inconvenient fluid is full of antiseptic enzymes that protect the lining of your sinuses while flushing out potentially dangerous invaders. It's a non-Newtonian gel perfectly adapted for trapping microbes right down to the molecular level. It's a hero of your immune system, right out on the front lines of the daily fight to keep you healthy. So show a little appreciation. Mucus is on your side.

Mucus is also so much more than the slimy stuff seeping from your nasal cavities. The term describes any viscous secretion (mucus is mostly water, mixed with glycoproteins and other molecules) that is produced by and covers the body's cavities and organs. Bony fish produce external mucus to protect against infectious agents. The southern sand octopus uses mucus to line the walls of its sandy burrow to keep the hole from collapsing. It's thought that earthworms use mucus to communicate and that dolphins rely on snot to make their distinctive clicks.

In humans, mucus serves all kinds of useful purposes. It protects the lining of your stomach from getting corroded by acid. It also acts as a lubricant in your esophagus, helping food pass down smoothly en route to your stomach.

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But your nose is where mucus really shines. That's where it works with stunning efficiency to filter the air you breathe, trapping dust particles, bacteria and other baddies before they get into your lungs.

“It's the initial defensive system of your body to the outside world,” said Aaron Pearlman, a rhinologist — or nose specialist — at the Weill Cornell Medicine and New York-Presbyterian.

Doctors estimate that the nose produces about four cups of mucus a day. The nose's particular type of slime is made of mostly water, with small amounts of mucin (a lubricating protein that helps prevent bacteria from clumping together in biofilms) and other particles. Its strands form a mesh, with small pores capable of trapping most bacteria, allergens and pollutants. Only the smallest viruses can slip through mucus's defenses.

Its color — green, white, yellow or brown — comes from the dead cells stuck in it. The lighter colors come from white blood cells, which fight infection (though, contrary to popular belief, green mucus is not necessarily evidence you should be on antibiotics). Orange or brown mucus is stained by dried up blood — an indication that the nose tissue is inflamed.

Nasal mucus has two elements: a thin, fluid layer on the bottom, called sol, and a thick, sticky layer evolved to trap invaders, called gel. When microscopic tentacles on the surface of your nasal passages, called cilia, beat against the sol, it and the gel layer on top are flushed out of the nose.

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If you find yourself sneezing and coughing a lot when you're sick, a virus may have done damage to your cilia, forcing the body to find other ways to get rid of mucus. David King, who teaches evidence-based medicine at the University of Queensland, in Australia, wrote in the Conversation that the best treatment for a stuffed-up nose is saline solution, which has been shown to improve cilia's beat frequency.

But the vast majority of mucus does not fly out your nose or mouth. Instead, your cilia push it to the back of your nose and throat, where your mucus is swallowed.

Grossed out? You shouldn't be. Your stomach is well-equipped to neutralize any infectious agents that might wind up there in your swallowed snot. Plus, mucus is probably a lot more sanitary than some other things you eat; scientists at MIT have proposed coating medical lab equipment with mucin because of its incredible antibacterial powers.

Canadian researcher Scott Napper has even suggested we should be intentionally eating our own boogers as a way to acquaint our immune systems with potential pathogens. Speaking to the Canadian Broadcasting Corp. in 2013, he noted that snot has an appealing taste — possibly an evolutionary signal that we should snack on it.

“I've got two beautiful daughters, and they spend an amazing amount of time with their fingers up their nose,” Napper said. “And without fail, it goes right into their mouth afterwards. Could they just be fulfilling what we're truly meant to do?”

We're just as happy leaving our mucus in a tissue — and your friends and family would probably appreciate if you do the same, at least while in public.

But Napper's broader point still stands: Mucus has an important role to play in your body. It's something to keep in mind the next time you sneeze.

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