You are literally covered in staph and strep and e coli, and all sorts of dubious characters that are intent on using you, and your body’s many resources, to feed themselves, find shelter, and reproduce as much as they want.

你全身佈滿金黃色葡萄球菌、鏈球菌、大腸桿菌，還有其他可疑的病菌，想要利用你和你身體的資源去滋養自己、找安身之處，再拼命繁殖。

And, hey, we all gotta make a living. But it is not your job to give these guys a free lunch.

嘿，大家都要維持生計，但你不需要給這些傢伙白吃的午餐。

So your body has developed a three-part policy toward these shady customers, and its enforcement is handled by your immune system.

因此你的身體發展出一套策略，分三部分去對付狡猾的病菌，由免疫系統負責執行。

The immune system is different from all the other systems we’ve talked about this year and that it’s not a specific, tissue-organ-system kind of system.

免疫系統有別於我們今年所談論過的其他所有系統，今天談的並非指特定組織、器官那種系統。

Instead, it involves a whole bunch of different tissue groups, organ systems, and specialized-but-widely-distributed defense cells.

而是包含一大堆不同的組織和器官，還有專業且分佈廣泛的防禦細胞。

Together, this league of extraordinary substances joins forces to perform all of the defense functions your body depends on to keep you alive in an incredibly germy world.

這個特殊軍團便聯合起來執行身體所仰賴的防禦功能讓你在充滿細菌的世界得以存活。

And the first line of defense in this never-ending battle?

這場打不完的仗它的防禦最前線是？

That’s your innate, or nonspecific, defense system.

那就是先天或不特定的防禦系統。

Like your average frontline soldier, it’s prepared to immediately engage with anyone suspicious, and it mostly includes stuff we were born with, like the external barricades of your skin and mucous membranes, and internal defenses like phagocytes, antimicrobial proteins, and other attack cells.

就如一般的前線士兵，隨時準備好要對付可疑的東西，防禦最前線通常包含我們天生就有的如皮膚表面的屏障與粘膜，以及內部的防禦包含吞噬細胞、抗菌蛋白以及其他的攻擊細胞

But some enemies must be fought with special forces. And here, your body can deploy your adaptive, or specific defense system, which is more like your Seal Team Six.

但對付特定的敵人需要特定的軍隊，身體可以部署會應變的或特定的防禦系統，更像是體內的美國海豹六隊

It takes more time to call in, but it’s specially designed to go after specific targets.

動用特種軍隊需要一些時間，但卻是設計用來對付特定敵人的。

And it keeps files on those bad guys so it knows how to handle them next time around.

它們會記錄曾入侵的病菌，如此才知道下次要如何對付他們。

But today we’re going to focus on your innate system, and look at how it uses an arsenal of physical and chemical barriers, killer cells, and even fever, to keep you healthy.

但今天要把重點放在你的先天性免疫系統，並看它如何運用身體和化學屏障或殺手細胞，甚至是發燒等機制來保持你的健康。

Proving that sometimes, the symptoms we associate with illness are actually the signs that we're healing.

這證明通常和生病聯想在一起的症狀是我們正在復原的徵兆。

Just because something is simple doesn’t mean that it can’t be elegant.

簡單的事情也可以高雅。

I mean, your body is capable of some incredibly sophisticated things, including defending itself from infection.

你的身體能夠進行一些複雜到令人難以置信的事情，包含免於感染。

But occasionally there’s something to be said for brute force.

但有時候也會出動蠻力部隊。

And a lot of your innate immune system’s functions aren’t exactly subtle.

而先天免疫系統的功能有很多其實也並不是那麼精細的。

For example, your body’s very first line of defense is a simple physical barrier.

例如，你身體的第一道防線是最簡單的物理防護。

And it works!

而且很有效！

Like a wall around a fortress, your skin does a fantastic job of keeping out all manner of malevolent microorganisms.

皮膚如城堡的外牆，把不懷好意的微生物擋在體外。

As long as that tough, keratinized epithelial membrane doesn’t get torn open or busted up too much, you could probably, like, make snowballs out of raw sewage and still be alright.

只要那層堅硬、角化的上皮膜不被撕開或裂得太大，你或許可以用未經處理的污水做雪球也無所謂。

Although...no. No.

雖然...算了當我沒說。

Your many mucous membranes also provide a handy physical barrier.

許多的粘膜也給予最佳的外在防護。

You’ll remember that they line any cavity that opens up into the germy outside world, including the respiratory, digestive, urinary, and reproductive tracts.

它們在身體各個向充滿細菌的世界門戶大開的通道處形成保護膜，包含呼吸道粘膜、消化道黏膜、尿道粘膜及生殖器官粘膜。

Not only do your skin and mucosa supply simple physical protection, they also pack some serious chemical weaponry.

你的皮膚和粘膜不只提供簡單的物理保護，同時還提供了強大的化學武器。

Eat some questionable leftovers for lunch?

吃下疑似有問題的午餐？

Don’t worry, your stomach is literally filled with acid, so you probably are covered.

別擔心，你的胃基本上佈滿胃酸，問題迎刃而解。

Walk face-first into your co-worker’s nasty sneeze cloud?

一頭撞進你同事的噴嚏雲霧裡嗎？

No worries, your nasal passages can whip up a tissue-box worth of sticky mucus to help trap viruses before they enter your lungs.

別緊張，你的鼻腔可以在病毒進到肺之前，製造出可以擤完一盒面紙的黏膜來阻止它進入。

You’ve also got bacteria-fighting enzymes in your saliva and lacrimal eye fluid, and peptides called defensins in your skin and membranes that help keep bacteria and fungi from setting up shop around inflamed or scraped skin.

你的口水、淚腺有抗菌的酵素，皮膚和粘膜裡的多肽防禦素防止細菌與真菌在發炎或脫皮的皮膚周圍逗留。

Which, no matter how careful you are, you’re gonna get, one way or another.

不管你有多小心，總會遇到這些情況。

Maybe you shave with a dull blade or you just brush your teeth too hard.

也許你的除毛刀不利、刷牙太過用力。

And don't get me started about the dangers of bagel-cutting.

更別提切貝果有多危險。

So when you’ve breached that first, simple line of defense, it’s time to call on your second line of internal innate defenses.

當第一道、簡單防線被衝破，是時候請出第二道的先天性內部防禦機制。

This is where your body starts pulling strategic maneuvers like firing up a fever, releasing chemical signals, causing inflammation, or other defensive tactics that help identify and attack infectious invaders.

這時候你的身體就會開始策略性應戰，像是發燒、釋放化學訊號、發炎或其他防禦策略，以協助辨識、攻擊入侵的感染原。

Some of the first defensive cells on the scene are your phagocytes.

第一個到場的防禦細胞中，有些是噬菌細胞 (phagocytes)。

Their name literally means “to eat,” and like Pac-Man, they indiscriminately chase down intruders and gobble them up.

字面上的意思為「吃」，像小精靈一樣，只要是入侵者都緊追在後，並將其吞下肚。

And they come in a few different varieties:

而噬菌細胞有數種：

First, you’ve got neutrophils, which are the most abundant type of your white blood cells.

首先有嗜中性白血球，是白血球中數量最多的。

They kind of self-destruct after devouring a pathogen.

他們在吞下病原體後會自我毀滅，而事實上你已經見過。

And, in fact, you’ve probably seen piles of their little dead bodies, because that’s what pus is made of.

它們成堆的小小屍體，那就是膿。

But the bigger, tougher phagocytes are the macrophages.

更大、更強壯的噬菌細胞是巨噬細胞。

They’re derived from monocyte white-blood cells that have moved out of the blood stream to occupy tissues.

它們變化自竄出血管、佔據組織的單核白血球細胞。

And some are free types that patrol tissues looking for creepers, while others are fixed -- attached to fibers in specific organs, devouring anything suspicious that passes by.

有些可以自由在組織內移動尋找病菌，有些則是在定點－－固定在特定器官的纖維上，吞下經過的任何可疑對象。

So when a macrophage in, say, the finger I just cut slicing a bagel, sees a new bacterium coming along, it snares it using cytoplasmic extensions, reels it in, completely engulfs it, and -- essentially -- digests it and spits the rest out.

假設今天切貝果時手指受傷，巨噬細胞看見新的細菌出現，就會以偽足捕捉、捲進內部並完全吞噬，最後消化它並吐出殘餘。

And unlike neutrophils, it can do this over and over again, like a boss.

不同於嗜中性白血球的是，巨噬細胞可以不斷重複吞噬病菌，超霸氣。

But not all your defense cells are phagocytic.

但並非所有的防禦細胞都是噬菌細胞。

You’ve also got cells with what is by far the awesomest name of any cell in the body: the natural killer cells.

也有目前體內名字最威的細胞：自然殺手細胞。

You can call them NK cells if you want to, but like, why would you do that?

你想要的話，也可以叫它「NK 細胞」，但還是全名比較威。

Anyway, these tiny assassins patrol your blood and lymph looking for abnormal cells, and are unique in that they can kill your own cells if they are infected with viruses or have become cancerous.

這些小小殺手在血液和淋巴裡巡邏，找尋不正常的細胞，特別的是，它們可以殺死體內受病毒感染，或轉為惡性的細胞。

How can they tell?

他們又是如何分辨的呢？

A normal, healthy cell contains a special protein on its surface called MHC1, or major Histocompatibility Complex.

一個正常健康的細胞表面有種特別的蛋白質叫做 MHC1，或叫主要組織相容性複合體。

But if it’s infected, it stops making that protein.

若受感染則無法製造那種特殊的蛋白質。

And if an NK cell detects a defective cell, it doesn’t swallow it whole like a macrophage -- it pokes it with an enzyme that triggers apoptosis, or programmed cell death, which is pretty awesome.

如果一個 NK 細胞偵測到缺陷細胞，不會像巨噬細胞那樣吞下去，而是將酵素刺入缺陷細胞並啟動細胞凋亡，或稱細胞程序性死亡，太棒了！

So those are some ways your innate immune cells handle their enemies, but how do they know where to look in the first place?

所以這是一些先天性免疫細胞對付敵人的方式，但它們一開始怎麼知道要往哪裡找？

So, let’s talk strategery.

就讓我們來談談策略吧！

So, say you’re in a banana factory and you slip on a banana peel and scrape your knees.

假如你今天在香蕉工廠踩到香蕉皮滑倒，膝蓋破皮。

Your outer fortress has been breached, and the pathogens are just flooding in like orcs through Helm’s Deep.

你的外部城牆已被攻破，病原體正如獸人從聖盔谷源源不絕地竄入 (註：魔戒中的戰爭場景)。

Banana factories are very dirty places.

香蕉工廠是很髒的。

Now your body wants to contain the spread of pathogens, clean up the mess, and get healing as quickly as possible, so it cues up your inflammatory response.

現在你的身體想控制病原體的擴散，清理現場並盡快復原，便有了發炎的反應。

This is basically an internal fire alarm, only it uses chemicals instead of sirens to get the message across, and instead of smoke and fire you sense redness, swelling, heat, and pain.

這基本上是體內的警報器，只是傳遞訊息的方式不是警鈴大作，也不是火焰和黑煙，而是透過化學物質傳遞訊息，並引起發紅、腫脹、灼熱及疼痛。

For example, in the event of injury, specialized mast cells in your connective tissue send out histamine molecules.

舉例來說，受傷時，在連接組織中的特定肥大細胞會釋出組織胺。

And histamine is great at calling in the cavalry.

而組織氨是很會呼叫騎兵的。

For one thing, it causes vasodilation, which creates redness and heat at the site of the injury.

它使得血管舒張，造成傷口發紅、灼熱感。

Now, those things might freak you out a little, but they’re actually signs of healing -- the increased temperature, for example, ratchets up the cells' metabolic rates so they can repair themselves faster.

這些症狀可能會讓你有點嚇到，但實際上卻是復原的徵兆，例如，上升的體溫提高了細胞的新陳代謝率，如此一來細胞就能以較快的速度自我修復。

Meanwhile, histamines and other inflammatory chemicals also increase the permeability of blood vessels, causing nearby capillaries to release protein-rich fluids.

同時，組織氨和其他發炎的化學物質也增加了血管的滲透性，使得周圍的微血管釋放富含蛋白質的液體。

This causes swelling -- which again, is actually a good thing -- because that leaked protein helps clot blood and form scabs, while the lymphatic system sucks up and filters that extra fluid, cleaning it up before putting it back into your bloodstream.

而這便導致腫脹，但再次聲明這是好事，因為那些釋放出來的蛋白質可以幫助凝血、結痂，而淋巴系統會同時吸收並過濾那些液體，清理過後再送回到血液中。

And of course, like chum to sharks, an inflamed knee is also going to attract a bunch of local phagocytes -- which find it easier to escape your now-leaky capillaries -- and lymphocytes that are also flowing freely, helping to destroy pathogens while also cleaning up dead-cell wreckage.

當然，就像鯊魚餌對鯊魚一樣，發炎的膝蓋會吸引大批患部的噬菌細胞，它們這時正好方便從滲漏的微血管中出來，同時，也會引來自由移動的淋巴球協助消滅病原體，並清理死細胞的殘骸。

And don't forget: During all this, the neutrophils have been doing their best, but they were the first wave to arrive, so by this time, they’re starting to die in heaps.

別忘了，過程中嗜中性白血球也盡了全力作戰，但因為他們是第一批趕到的，到了這個時候，它們已經開始成堆地死亡了。

They're triggered when the injured knee-skin cells release chemicals that begin leukocytosis – the release of neutrophils from the bone marrow where they're made into the bloodstream.

它們在受傷的膝蓋皮膚釋放化學物質並使白血球增多時，就已經被驅動了，白血球增多即是從骨髓釋放嗜中性白血球至血液中。

To attract the neutrophils to the damaged area, inflamed endothelial cells in the capillaries send out chemicals that act like homing devices-- and when the neutrophils arrive, they cling to the capillary walls near the injury, flatten themselves out and squeeze through the vessel walls to get to work.

為了將嗜中性白血球引到受傷區域，微血管中發炎的內皮細胞會釋出像是導引裝置的化學物質，當嗜中性白血球到達時，它們會附在靠近患部的微血管壁上，壓平、擠壓自己穿過血管壁，便開始吞噬病菌。

Your big monocytes eventually roll up to the battle, and transform into hungry macrophages, replacing that first line of now-dead neutrophils and basically just eating up any lingering enemies and then cleaning up the carnage.

大單核細胞最後也會聚集加入戰役，並轉變成飢餓的巨噬細胞，去取代現在已陣亡的第一線嗜中性白血球，接著吃掉任何正在遊蕩的敵人並清理屠殺現場。

Now, all this works pretty well in most circumstances.

以上這些機制在大多數時候都管用。

But you may have noticed with a more major injury, or an especially nasty virus or infection, sometimes your local troops get overrun.

但你可能會在你受傷較嚴重或正在對抗強大的病毒或感染的時候才會注意到，有時候你的大軍就可能被敵方殲滅。

When white blood cells and macrophages run into more foreign invaders than they can handle, they let loose pyrogen chemicals that tap the hypothalamus and raise your body’s thermostat, calling in a systemic fever to burn everything.

當白血球細胞與巨噬細胞面臨超過負荷量的陌生敵人，他們釋出致熱化學物質刺激視床下部並提高體溫，使人全身發燙，以燒掉入侵者。

The resulting temperature rise increases the metabolism of your cells so they can heal faster, and it also tells the liver and spleen to hold onto all of their iron and zinc, so those things can’t contribute to bacterial growth.

增溫提高細胞的新陳代謝率，加快其復原速度，同時也讓肝脾不再釋放鐵與鋅，才不會幫助病菌生長。

But even then, sometimes, well sometimes you find yourself facing a more formidable foe.

但即使如此，有時候我們會發現自己的確是大敵當前。

That’s when you call in the specialists -- your adaptive immune defenses.

這時候就需要專業的來，也就是你的後天免疫系統。

And to learn exactly how they save the day, you have to watch next time.

欲知後天免疫如何殺敵，且聽下回分解。

But for now you learned that your immune system’s responses begin with physical barriers like skin and mucous membranes, and when they’re not enough, there are your phagocytes -- the neutrophils and macrophages.

現在你知道體內免疫系統是從皮膚、粘膜等外在屏障開始的如果被攻破，還有你的噬菌細胞：嗜中性白血球和巨噬細胞。

You also learned about natural killer cells and the inflammatory response, and watched as all of these elements saved the day when you slipped on a banana peel.

你也學到自然殺手細胞以及發炎的反應，還看了這些小兵在你踩到香蕉皮跌一跤後如何拯救你。

Thank you to our Headmaster of Learning, Linnea Boyev, and thank you to all of our Patreon patrons whose monthly contributions make Crash Course possible, not only for themselves, but for everybody.

感謝 Linnea Boyev，也謝謝 Patreon 的贊助者，你們每個月的贊助造就了 Crash Course 速成班，不是為自己，而是為每個人而作。

If you like Crash Course and you want to help us keep making videos like this, you can go to patreon.com/crashcourse.

如果你喜歡速成班，也想幫助我們繼續拍影片，請到 Patreon.com/crashcourse。

This episode was filmed in the Doctor Cheryl C. Kinney Crash Course Studio, it was written by Kathleen Yale, the script was edited by Blake de Pastino, and our consultant is Dr. Brandon Jackson.

本集在 Cheryl C. Kinney 博士速成班工作室拍攝，文字由 Kathleen Yale 撰寫，Blake de Pastino 編輯，我們的顧問是 Brandon Jackson 博士。

It was directed by Nicholas Jenkins, edited by Nicole Sweeney, our sound designer is Michael Aranda, and the Graphics team is Thought Cafe.

本集由 Nicholas Jenkins 執導，Nicole Sweeney 編輯，由 Michael Aranda 負責音效設計，Thought Cafe 負責圖表製作