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

到底黏液是什麼呢? 它除了讓你十分狼狽以外還有些什麼作用呢?

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

你的身體每天會產生一公升以上的黏液

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

身體中所有潮濕且未被皮膚包覆的器官

like your eyes, nose, mouth, lungs, and stomach get a liberal coating.

像是眼、鼻、嘴、肺以及胃都有大量黏液保護

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

因此才有「黏膜」一詞

Mucus plays lots of roles in your body.

黏液是身體中扮演了很多角色

It keeps delicate tissues from drying out and cracking,

它保護脆弱的組織不會因為太乾燥而受損

which would expose them to infection.

組織受損時可是會增加感染風險的

It lubricates your eyes so you can blink.

它濕潤你的眼睛，讓你得以眨眼

It protects your stomach lining from acid.

它保護你的胃不被胃酸侵蝕

It neutralizes threats by removing or trapping substances that could make you sick.

它黏住或直接清除一些可能會害你生病的物質

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

最後一點，它還是你體內數以兆計細菌的家

your microbiota, under control.

居於其中的微生物群落，盡在掌控之中

Mucus contains lots of different compounds,

黏液中含有許多不同的化合物質

including proteins, fats, and salts.

包括蛋白質、脂質以及鹽類

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

但其中一樣物質是黏液眾多功能的關鍵，那就是「黏素」

Mucins are the primary large molecules in mucus

黏素是黏液中主要的大分子物質

and are essential for giving mucus its slippery feel.

也是黏液之所以黏黏滑滑的原因

They belong to a class of proteins called glycoproteins

黏素是醣蛋白中的一員

which are built out of both amino acids and sugars.

由胺基酸以及醣類構成

In mucin, long chains of sugars are attached to specific amino acids in the protein backbone.

在黏素的結構中，糖鏈會接在蛋白質骨架的特定胺基酸上

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

親水性的糖鏈使黏素得以溶在富含水的體液當中

Mucus, which is up to 90% water,

而黏液當中有 90% 都是水分

stays hydrated thanks to these sugar chains.

黏素中的糖鏈也能幫忙抓住黏液中的水分

Some of these mucins can interact with other mucin molecules

黏素還會和其他黏素中的分子結合

to create a complex network that establishes a barrier against pathogens and other invaders.

成為更大而複雜的結構，進而得以擋住外來的病原和有害物質

That's why mucus is the body's first line of defense against foreign objects, like bacteria and dust.

因此黏液是身體阻擋細菌或灰塵等外來物質的第一道防線

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

我們的呼吸道會持續地分泌黏液，以清除這些有害物質

like a slimy conveyor belt.

就好像一條黏黏滑滑的輸送帶一樣

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

這可以避免細菌在纖弱的肺部滋生

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

或進入血液循環，造成嚴重感染

Many of those harmful bacteria also cause diseases

部分有害的病菌

when they cluster into slimy growths called biofilms.

會聚集成一種稱為生物膜(或菌膜)的群落，使你生病

But mucus contains mucins, antimicrobial peptides, antibodies,

然而，在黏液中含有黏素、抗微生物胜肽、抗體

and even bacteria-hungry viruses called bacteriophages

甚至是以細菌為食的病毒，也就是噬菌體

that all work together to prevent biofilms from forming.

上述物質共同作用後可以避免生物膜的產生

If microbes do become harmful and you get sick,

假如微生物已產生危害，使你生病

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

那麼身體就會增加黏液的產量，試圖將這些侵入者沖洗掉

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

並且免疫系統中的白血球會湧進黏液中

In fact, the greenish mucus often associated with infections

這也是感染時會出現偏綠色黏液的原因：

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

白血球所產生的酶會造成偏綠的顏色

This multi-pronged approach to bacterial management

這樣多管齊下的方式

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

是讓我們免於病痛纏身的主要原因之一

Even though mucus protects against the infectious bacteria,

即使黏液的作用是抵抗致病細菌

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

但事實上，絕大多數居住於你體內的細菌都是無害的

and many are actually beneficial.

甚至有許多是有益的

That's particularly true when they live in mucus,

尤其是那些住在黏液中的細菌

where they can perform important functions,

它們可以發揮一些重要的功能

like synthesizing vitamins, suppressing harmful inflammation,

像是製造維生素、抑制過度發炎反應

and controlling the growth of more harmful species.

以及抑制有害微生物的過度滋生

So even though you probably associate mucus with being ill,

因此，就算黏液總是讓你聯想到生病

it's really helping you stay healthy.

它其實是在幫助你保持健康

Sure, it might seem gross,

也許它看來很噁心

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

但你還能想到第二種物質能幫你保持濕潤、

keep your body clean, fight infection, and domesticate a teeming bacterial population?

保持體內清潔、對抗感染、以及管控微生物數量的嗎?

Nope, just mucus.

僅有黏液做得到