A mosquito lands on your arm, injects its chemicals into your skin, and begins to feed.
一只蚊子落到了你的手臂上，把化学物质注射到你的皮肤内，然后开始大快朵颐。
You wouldn't even know it was there, if not for the red lump that appears, accompanied by a telltale itch.
如果没有在叮咬后出现又红又痒的包，你甚至不会察觉它的存在。
It's a nuisance, but that bump is an important signal that you're protected by your immune system,
虽然很烦人，可这个包是一个是重要的信号，它表示你正被你的免疫系统保护着。
your body's major safeguard against infection, illness, and disease.
免疫系统是你身体的保镖，使你的身体免受感染和疾病的侵害。
This system is a vast network of cells, tissues, and organs that coordinate your body's defenses against any threats to your health.
免疫系统是由细胞、组织和器官形成的巨大网络，负责组织身体防御可能危害健康的外来入侵者。
Without it, you'd be exposed to billions of bacteria, viruses, and toxins
没有它，你将会暴露在数以亿计的细菌、病毒和毒素之中，
that could make something as minor as a paper cut or a seasonal cold fatal.
这些病原体能让一个小割伤或小感冒变得致命。
The immune system relies on millions of defensive white blood cells, also known as leukocytes, that originate in our bone marrow.
白细胞是免疫系统中一个不可或缺的成员，它们也被称作白血球，由我们的骨髓产生。
These cells migrate into the bloodstream and the lymphatic system, a network of vessels which helps clear bodily toxins and waste.
这些细胞在血液和淋巴系统中游荡，血液和淋巴系统是用于清除毒素和废物的管道系统。
Our bodies are teeming with leukocytes: there are between 4,000 and 11,000 in every microliter of blood.
我们的身体里有大量白血球，每微升血液中就有4000到11000个白血球。
As they move around, leukocytes work like security personnel, constantly screening the blood, tissues, and organs for suspicious signs.
它们在身体里的作用如同保安，持续不断地在血液、组织和器官中搜查可疑迹象。
This system mainly relies on cues called antigens.
免疫系统主要依靠抗原作为搜查病原体的线索。
These molecular traces on the surface of pathogens and other foreign substances betray the presence of invaders.
这些在病原体和外来物质表面上的分子痕迹标示着入侵者的存在。
As soon as the leukocytes detect them, it takes only minutes for the body's protective immune response to kick in.
白血球发现它们之后，只需要几分钟，身体的免疫反应就会被触发。
Threats to our bodies are hugely variable, so the immune response has to be equally adaptable.
对我们有威胁的物质种类繁多，所以免疫反应必须随机应变。
That means relying on many different types of leukocytes to tackle threats in different ways.
这意味着，要依靠许多不同种类的白血球，通过不同的方式抵御入侵者。
Despite this diversity, we classify leukocytes in two main cellular groups, which coordinate a two-pronged attack.
尽管白血球种类繁多，我们仍可以把它们分为两个主要细胞组，它们能双管齐下地攻击敌人。
First, phagocytes trigger the immune response by sending macrophages and dendritic cells into the blood.
首先，吞噬细胞输送巨噬细胞和树突状细胞到血液里，进而触发免疫反应。
As these circulate, they destroy any foreign cells they encounter, simply by consuming them.
这些细胞在血液循环中会通过吞噬的方法将遇到的外来细胞消灭。
That allows phagocytes to identify the antigen on the invaders they just ingested
这让吞噬细胞能够辨别入侵者身上携带的抗原，
and transmit this information to the second major cell group orchestrating the defense, the lymphocytes.
然后把信息传送给第二个主要细胞组，即淋巴细胞，它们负责组织身体的防御。
A group of lymphocyte cells called T-cells go in search of infected body cells and swiftly kill them off.
一群叫做“T细胞”的淋巴细胞开始搜索被感染的身体细胞，然后迅速地消灭它们。

Meanwhile, B-cells and helper T-cells use the information gathered from the unique antigens
与此同时，B细胞和辅助T细胞使用从抗原处收集的信息
to start producing special proteins called antibodies.
来制造被称为“抗体”的特殊蛋白质。
This is the pièce de résistance: Each antigen has a unique,
它们可不容小觑：每个抗原都有一个独特抗体附着其上，
matching antibody that can latch onto it like a lock and key, and destroy the invading cells.
与之相匹配，就像锁和钥匙一样，能够摧毁入侵细胞。
B-cells can produce millions of these,
B细胞能生产数以百万计的抗体，
which then cycle through the body and attack the invaders until the worst of the threat is neutralized.
这些抗体在身体里循环，并攻击入侵者，直到解除对身体的威胁。
While all of this is going on, familiar symptoms, like high temperatures and swelling,
与此同时，令人熟悉的症状，比如发烧和肿胀，
are actually processes designed to aid the immune response.
其实是为了帮助免疫反应而产生的。
A warmer body makes it harder for bacteria and viruses to reproduce and spread because they're temperature-sensitive.
较高的体温能够使细菌和病毒更难繁殖和扩散，因为它们对温度很敏感。
And when body cells are damaged, they release chemicals that make fluid leak into the surrounding tissues, causing swelling.
当身体细胞受损时，它们会释放一种能让体液渗入周围组织的化学物质，从而造成肿胀。
That also attracts phagocytes, which consume the invaders and the damaged cells.
这也会吸引吞噬细胞来吞食入侵者和受伤的细胞。
Usually, an immune response will eradicate a threat within a few days.
通常来说，免疫反应能在几天内根除威胁。
It won't always stop you from getting ill, but that's not its purpose.
它不会总让你免受疾病之苦，那并不是它的职责。
Its actual job is to stop a threat from escalating to dangerous levels inside your body.
它真正的职责是阻止威胁在你的身体中上升到危险的程度。
And through constant surveillance over time, the immune system provides another benefit: it helps us develop long-term immunity.
另外，通过对身体持续不断的监视，免疫系统还能提供另一个好处：那就是帮助我们构建长期免疫力。
When B- and T-cells identify antigens, they can use that information to recognize invaders in the future.
当B细胞和T细胞识别出抗原，它们能使用这些信息识别未来的入侵者。
So, when a threat revisits, the cells can swiftly deploy the right antibodies to tackle it before it affects any more cells.
所以，当相同威胁再次到来时，这些细胞就能快速地调动相应的抗体，在感染扩散之前进行拦截。
That's how you can develop immunity to certain diseases, like chickenpox. It doesn't always work so well.
这就是我们能对水痘等特定疾病产生免疫力的原因。但这并不总能一劳永逸。
Some people have autoimmune diseases, which trick the immune system into attacking the body's own perfectly healthy cells.
一些人患有自体免疫疾病，他们的免疫系统会变得敌我不分，并开始攻击正常而健康的身体细胞。
No one knows exactly what causes them, but these disorders sabotage the immune system to varying degrees,
这种病的病因不得而知，但是这些疾病会在不同程度上影响免疫系统，
and underlie problems like arthritis, Type I diabetes, and multiple sclerosis.
并引发像关节炎，1型糖尿病和多发性硬化症等问题。
For most individuals, however, a healthy immune system
不过，对于大多数人来说，健康的免疫系统
will successfully fight off an estimated 300 colds and innumerable other potential infections over the course of a lifetime.
能在一生中成功抵御大约300次感冒和不计其数的其他潜在感染。
Without it, those threats would escalate into something far more dangerous.
没有免疫系统，这些威胁就会发展成更危险的疾病。
So the next time you catch a cold or scratch a mosquito bite, think of the immune system. We owe it our lives.
所以，你下次得感冒或被蚊子咬时，想想你的免疫系统，我们的生命全靠它保障。