injects its chemicals into your skin, and begins to feed.

將化學物質注入你的皮膚，並開始以你的鮮血為食

You wouldn't even know it was there, if not for the red lump that appears,

如果不是那伴隨著陣陣搔癢的紅色腫包

accompanied by a telltale itch.

你甚至不會知道蚊子的存在

It's a nuisance, but that bump is an important signal

雖然發癢的腫包很討厭，但這卻是你正受到

that you're protected by your immune system,

免疫系統保護的重要訊號

your body's major safeguard against infection, illness, and disease.

免疫系統是你的身體預防疾病或傳染病的主要保護措施

This system is a vast network of cells, tissues, and organs

這個系統是一個由細胞、組織和器官組成的巨大網絡

that coordinate your body's defenses against any threats to your health.

它們會互相協調身體的防禦機制，以預防對健康的任何威脅

Without it, you'd be exposed to billions of bacteria, viruses, and toxins

若沒有免疫系統，你將會暴露在數十億細菌、病毒與毒素之中

that could make something as minor as a paper cut or a seasonal cold fatal.

輕則造成如紙張劃傷般的影響，重則遭受致命症狀，如季節性感冒

The immune system relies on millions of defensive white blood cells,

免疫系統依靠著血液中數百萬具防衛功能的白色細胞─

also known as leukocytes,

也就是所謂的白血球來運作

that originate in our bone marrow.

白血球源自我們的骨髓

These cells migrate into the bloodstream and the lymphatic system,

它們會在血液和淋巴系統中移動

a network of vessels which helps clear bodily toxins and waste.

淋巴系統是一個負責清除身體毒素和廢棄物的導管系統

Our bodies are teeming with leukocytes:

我們的體內充滿了白血球：

there are between 4,000 and 11,000 in every microliter of blood.

在每微升血液中，就有 4000 至 1,1000 顆白血球

As they move around, leukocytes work like security personnel,

由於白血球們會到處移動，它們的工作就像保全一樣

constantly screening the blood, tissues, and organs for suspicious signs.

持續搜查血液、組織和器官中有無可疑的徵兆

This system mainly relies on cues called antigens.

免疫系統主要依賴一種名為「抗原」的線索來做判斷

These molecular traces on the surface of pathogens and other foreign substances

這些分子會追蹤病源體的表面以及其他

betray the presence of invaders.

會出賣侵略者存在的外來物質

As soon as the leukocytes detect them,

一但白血球偵測到它們

it takes only minutes for the body's protective immune response to kick in.

身體的免疫反應即會在數分鐘內見效

Threats to our bodies are hugely variable,

由於身體受威脅的種類繁多

so the immune response has to be equally adaptable.

所以免疫反應也要適應這些大量的威脅

That means relying on many different types of leukocytes

也就是我們需要依靠不同類型的白血球

to tackle threats in different ways.

以不同方法來處理這些威脅

Despite this diversity, we classify leukocytes in two main cellular groups,

依照它們的多樣性，我們將白血球細胞分成主要的兩組

which coordinate a two-pronged attack.

它們能互相協調出雙管齊下的攻擊

First, phagocytes trigger the immune response

首先，噬菌細胞會將巨噬細胞和樹狀細胞

by sending macrophages and dendritic cells into the blood.

送進血液中以觸發免疫反應

As these circulate, they destroy any foreign cells they encounter,

這些細胞會在體內循環，並以吞噬的方式摧毀任何

simply by consuming them.

它們遇見的外來細胞

That allows phagocytes to identify

噬菌細胞隨後會辨別

the antigen on the invaders they just ingested

他們剛吞下的侵略者身上的抗原

and transmit this information to the second major cell group

並把抗原資訊傳遞給負責策劃防禦行動的

orchestrating the defense,

第二組細胞─

the lymphocytes.

淋巴細胞

A group of lymphocyte cells called T-cells go in search of infected body cells

此時淋巴細胞中的 T 細胞們會找出受感染的體細胞

and swiftly kill them off.

並立刻除掉它們

Meanwhile, B-cells and helper T-cells

同時，淋巴細胞中的 B 細胞和輔助 T 細胞

use the information gathered from the unique antigens

會依照剛剛的特殊抗原收集到的資訊

to start producing special proteins called antibodies.

來產生特殊的蛋白質─也就是抗體

This is the pièce de résistance:

重頭戲來了：

Each antigen has a unique, matching antibody that can latch onto it

每個抗原都有一個獨特，可以相互吻合的抗體

like a lock and key,

它們就像鎖和鑰匙一樣

and destroy the invading cells.

當兩者結合後，抗體就能消滅帶有此抗原的侵略細胞

B-cells can produce millions of these,

B 細胞能夠製造數百萬的抗體

which then cycle through the body and attack the invaders

它們會持續在體內循環並攻擊侵略者

until the worst of the threat is neutralized.

直到最糟的威脅被解除

While all of this is going on, familiar symptoms,

當上述過程發生時，常見的症狀

like high temperatures and swelling,

如發燒和紅腫

are actually processes designed to aid the immune response.

這些過程是為了幫助免疫反應的運作

A warmer body makes it harder for bacteria and viruses to reproduce and spread

發熱的身體讓細菌和病毒難以在體內繁殖與散佈

because they're temperature-sensitive.

因為它們對溫度很敏感

And when body cells are damaged,

而當體細胞受損時

they release chemicals that make fluid leak into the surrounding tissues,

它們也會釋放出讓胞內液流入周圍組織的化學物質

causing swelling.

這會導致紅腫

That also attracts phagocytes,

此化學物質也會吸引噬菌細胞

which consume the invaders and the damaged cells.

它會一併吃掉侵略者和受損的體細胞

Usually, an immune response will eradicate a threat within a few days.

通常，免疫系統會花數天來根除威脅

It won't always stop you from getting ill, but that's not its purpose.

這期間可能會讓你生病，但這不是它的本意

Its actual job is to stop a threat

它的本意是為了遏止體內的威脅

from escalating to dangerous levels inside your body.

擴大到危險的階段

And through constant surveillance over time,

隨著免疫系統持續監察

the immune system provides another benefit:

它們也帶來了另一個益處—

it helps us develop long-term immunity.

幫助我們發展長期免疫

When B- and T-cells identify antigens,

一旦 B 細胞和 T 細胞辨識出侵略者的抗原

they can use that information to recognize invaders in the future.

它們未來就能再次以此資訊辨識侵略者

So, when a threat revisits, the cells can swiftly deploy the right antibodies

因此，當相同的威脅再次出現，它們可以即刻配對出與之符合的抗體

to tackle it before it affects any more cells.

並對付它們，以防它們再影響更多正常細胞

That's how you can develop immunity to certain diseases, like chickenpox.

這就是為何你能對特定疾病展現免疫的原因，例如水痘

It doesn't always work so well.

但這個機制並非總是能正常運作

Some people have autoimmune diseases,

因為有些人擁有「自體免疫性疾病」

which trick the immune system

這個病症會讓

into attacking the body's own perfectly healthy cells.

免疫系統攻擊自身的正常細胞

No one knows exactly what causes them,

沒人知道導致這個病症的原因

but these disorders sabotage the immune system to varying degrees,

但這類的系統失調除了造成免疫系統不同程度的破壞

and underlie problems like arthritis,

也導致了一些疾病的發生，如關節炎

Type 1 diabetes,

1 型糖尿病

and multiple sclerosis.

以及多發性硬化症

For most individuals, however,

對大多數人來說

a healthy immune system will successfully fight off an estimated 300 colds

一個正常的免疫系統，在我們一生中能成功擊退約 300 次感冒

and innumerable other potential infections over the course of a lifetime.

以及無數次的潛在傳染病

Without it, those threats would escalate into something far more dangerous.

若沒有它們的保護，這些威脅可能擴大到更危險的地步

So the next time you catch a cold or scratch a mosquito bite,

所以下次當你感冒或是搔抓著蚊子叮咬的腫包時

think of the immune system.

記得想起我們的免疫系統

We owe it our lives.

我們能好好生活都要感謝它