Immunity
A-Level Biology Topic 11 8:37 English narration · English + 中文 subtitles burned in
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Transcript
Catch measles once and you will almost certainly never catch it again.
得过一次麻疹，你几乎肯定不会再得第二次。
Your body has not become tougher, and the virus has not become weaker.
你的身体并没有变强壮，病毒也没有变弱。
What has changed is that your body now remembers.
改变的是：你的身体现在记住了它。
Look at this graph.
看这张图。
The first time the pathogen arrives, the response is small and slow, and you get ill.
病原体第一次到来时，反应又小又慢，你就病了。
The second time, the response is enormous and almost instant, and the pathogen is gone before you notice.
第二次，反应巨大而且几乎是瞬间的，病原体在你察觉之前就被清除了。
In this lesson we find out what does the remembering, and how we can give a body that memory on purpose.
这节课我们就来看看是什么在记忆，以及我们怎样有意地给身体这种记忆。
Watch the very first line of defence at work.
来看第一道防线是怎么工作的。
This white blood cell has found something foreign.
这个白细胞发现了外来的东西。
It flows around it, wraps it completely, and takes it inside in a little bag called a vesicle.
它绕过去把它整个包起来，装进一个叫小泡的小袋子带进细胞内。
Then enzymes are poured in, and the pathogen is digested.
然后倒进酶，病原体就被消化了。
No memory is needed for any of this, and no matching.
这整个过程不需要记忆，也不需要配对。
The cell simply destroys anything that does not belong.
这种细胞只是单纯地摧毁一切不属于身体的东西。
The cells that do this are called phagocytes, and you should be able to name two of them: macrophages and neutrophils.
做这件事的细胞叫吞噬细胞，你应该能说出其中两种：巨噬细胞和中性粒细胞。 这个过程有三步。
The process is called phagocytosis, and it has three steps.
首先它吞入病原体，把它完全包住。
First it engulfs the pathogen, surrounding it completely.
然后它用小泡里的酶把它消化掉。
Then it digests it, using enzymes inside the vesicle. And here is the step that matters most for what comes next. Afterwards, a macrophage keeps some parts of the pathogen and it displays them on its own surface, like holding up a wanted poster for the rest of the immune system to read.
接下来这一步对后面最重要： 之后巨噬细胞会留下病原体的一些部分，并把它们展示在自己的表面上， 就像举起一张通缉令，让免疫系统的其他细胞来读。
What is being displayed is an antigen.
被展示出来的东西叫抗原。
An antigen is a molecule, usually a protein, that sits on a cell surface where the immune system can recognise it.
抗原是一种分子，通常是蛋白质， 位于细胞表面，免疫系统可以识别它。
Now, every cell you own carries antigens too, so the immune system has to tell the difference.
而你自己的每个细胞也带着抗原， 所以免疫系统必须分辨得出差别。
Self antigens are your own markers, and the immune system learns to ignore them completely.
自身抗原是你自己的标记，免疫系统学会完全忽略它们。
Non-self antigens are foreign — the markers on a pathogen, for example — and those trigger a response.
非自身抗原是外来的——比如病原体上的标记——它们会触发反应。
Getting this wrong in either direction is serious. Ignore a pathogen and you get ill; attack yourself and you get an autoimmune disease.
两个方向弄错了都很严重：忽略病原体你就会生病；攻击自己就成了自身免疫病。
Here is the chain of events the first time a new pathogen arrives.
新病原体第一次到来时，事情是这样一连串发生的。
Step one, the macrophage displays the antigen it took from the pathogen.
第一步，巨噬细胞展示它从病原体上取下的抗原。
Step two, T-helper cells that recognise that particular antigen become active, and release chemicals that switch other cells on.
第二步，能识别这个特定抗原的辅助性T细胞被激活，并释放化学物质去开启其他细胞。
Step three, and this is the clever part.
第三步，也是巧妙的地方。
Your body already carries millions of different B-lymphocytes, each with a slightly different shape.
你的身体里本来就有几百万种不同的B淋巴细胞，各自形状略有不同。
Only the one whose shape matches this antigen is selected.
只有形状与这个抗原匹配的那一种被选中。
That one is then cloned, and it divides into two kinds of cell. Plasma cells, which pour out antibodies right now. And memory cells, which do nothing at all today.
那一种随后被克隆，并分化成两类细胞： 浆细胞，现在就大量分泌抗体；还有记忆细胞，今天什么也不做。
Meanwhile T-killer cells deal with a different problem: they destroy your own cells that the pathogen has already got inside.
与此同时，杀伤性T细胞处理另一个问题：它们摧毁那些已经被病原体侵入的你自己的细胞。
Now look at what the memory cells are for.
现在看看记忆细胞是干什么的。
On the graph, the first time is the primary response — that small, slow rise.
在图上，第一次是那条又小又慢的上升。
It takes days to build, and in those days you feel ill.
它要花好几天才建立起来，而这几天里你会觉得不舒服。
But the memory cells stay in the body long after the infection is over — for years, often for life.
但记忆细胞在感染结束后很久仍留在体内——好多年，往往是一辈子。
So if the same pathogen again enters, they are already there, already matched, and ready to go.
所以如果同一种病原体再次进入，它们已经在那里，已经配好，随时可以行动。
The secondary response is both faster and larger, and the pathogen is destroyed before it can make you ill at all.
第二次反应更快也更强，病原体在能让你生病之前就被清除了。
That is what long-term immunity actually is.
这才是长期免疫的真正含义。
And notice the catch. It works only against the same pathogen.
注意这里有个限制：它只对同一种病原体有效。
A new one starts the slow process all over again.
换一种新的，慢慢的过程又要从头来一遍。
So what is an antibody?
那么抗体是什么呢？
It is a Y-shaped protein made by the plasma cells.
它是浆细胞制造的一种 Y 形蛋白质。
The two tips of the Y are the antigen-binding sites, and the special shape at each tip is called the variable region.
Y 的两个末端是抗原结合位点，每个末端上那个特殊的形状叫可变区。
That shape fits one antigen and one only, like a lock and a key, which is why an antibody made against measles does nothing at all against flu.
这个形状只与一种抗原相配，像锁和钥匙一样， 这就是为什么针对麻疹产生的抗体对流感毫无作用。
Antibodies then help in several ways.
抗体接着以几种方式帮忙。
They stick to the antigens.
它们粘住抗原。
They clump pathogens together so they cannot spread, which is called agglutination.
它们把病原体聚成一团，使其无法扩散，这叫凝集。
They mark the pathogen so that phagocytes find it quickly.
它们标记病原体，让吞噬细胞很快找到它。
And they block harmful toxins, which is called neutralisation.
它们还封住有害的毒素，这叫中和。
Because one antibody binds one thing and nothing else, they make superb tools — if you can produce enough identical ones.
因为一种抗体只结合一种东西、别的都不结合，它们是极好的工具—— 只要你能生产出足够多完全相同的抗体。
That is what a monoclonal antibody is: one single type, all exactly the same.
单克隆抗体就是这个意思：单一一种，全都一模一样。
They are made by the hybridoma method.
它们用杂交瘤方法制造。
First an animal is given the antigen, so it makes B-lymphocytes producing the antibody we want.
首先给一只动物注入抗原，让它产生能制造我们想要的抗体的B淋巴细胞。
Those B-lymphocytes are then fused with a tumour cell, because a tumour cell divides without ever stopping.
然后把这些B淋巴细胞与肿瘤细胞融合，因为肿瘤细胞可以不停地分裂。
The fused cell is called a hybridoma, and it does both jobs at once: it makes the antibody, and it divides for ever.
融合后的细胞叫杂交瘤，它同时做两件事：制造抗体，并且永远分裂下去。
Learn the two uses.
记住两种用途。
In diagnosis, they detect one specific molecule — a pregnancy test uses them to find the hormone hCG.
在诊断上，它们检测某一种特定分子——验孕棒就用它们来找人绒毛膜促性腺激素。
In treatment, they carry a drug straight to target cells, such as cancer cells.
在治疗上，它们把药物直接送到目标细胞，比如癌细胞。
Immunity comes in four combinations, and you can get all four right by asking two questions.
免疫有四种组合，而问两个问题就能把四种都答对。
Question one: did the person make the antibodies themselves?
问题一：这个人自己制造了抗体吗？
Active immunity means yes — their own body met the antigen and produced its own antibodies and memory cells.
主动免疫的意思是"是"—— 他们自己的身体遇到了抗原，产生了自己的抗体和记忆细胞。
It is slow to start but it lasts.
它起效慢，但持久。
Passive immunity means no — ready-made antibodies were handed to them from outside.
被动免疫的意思是"否"——现成的抗体是从外面交给他们的。
It works immediately but it does not last.
它立刻见效，但不持久。
Question two: did it happen by chance, or on purpose?
问题二：这是偶然发生的，还是有意为之？
By chance is natural, on purpose is artificial.
偶然是天然的，有意是人工的。
Put the two answers together and you get the four boxes.
把两个答案组合起来，就得到四个方格。
And remember the key difference: only active immunity makes memory cells.
记住关键差别：只有主动免疫产生记忆细胞。
That single fact explains why passive immunity is immediate but short-lived.
仅这一条就解释了为什么被动免疫见效快却短暂。
Vaccination is artificial active immunity, done deliberately.
疫苗接种是有意进行的人工主动免疫。
A vaccine contains antigens — often a dead or weakened pathogen, or just one part of it.
疫苗里含有抗原—— 通常是死掉的或减弱的病原体，或者只是它的一部分。
Those antigens trigger a primary response exactly as a real infection would, so memory cells are made.
这些抗原会像真正的感染一样触发初次免疫反应，于是记忆细胞产生了。
But because the pathogen is dead or weakened, you never become ill.
但因为病原体是死的或被减弱的，你不会生病。
You get the memory without the disease.
你得到了记忆，却没有得病。
Then something larger happens.
接着会发生更大的事。
If enough people in a population are vaccinated, the pathogen cannot pass easily from person to person, because almost everyone it meets is already immune.
如果一个群体里有足够多的人接种了疫苗， 病原体就很难在人与人之间传播，因为它遇到的几乎每个人都已经免疫。
So it cannot reach the few who were not vaccinated.
于是它够不到那少数没有接种的人。
That protection of the unvaccinated is called herd immunity.
这种对未接种者的保护叫群体免疫。
Let us classify four cases using the same two questions.
我们用同样的两个问题给四种情况分类。
A baby receives antibodies in breast milk.
婴儿从母乳中获得抗体。
The baby did not make them, so passive; and it happened naturally, so natural passive.
婴儿不是自己造的，所以是被动；而且是自然发生的，所以是天然被动。
A child is vaccinated against measles.
一个孩子接种了麻疹疫苗。
The vaccine makes the child produce their own antibodies, so active; and it was done deliberately, so artificial active.
疫苗让孩子产生自己的抗体，所以是主动； 而且是有意进行的，所以是人工主动。
Someone recovers from chickenpox.
有人从水痘中康复。
They made their own antibodies during the infection, so active; and it happened by chance, so natural active.
他们在感染期间产生了自己的抗体，所以是主动；而且是偶然发生的，所以是天然主动。
And a snakebite patient is given antivenom.
还有被蛇咬的病人被注射抗蛇毒血清。
Those antibodies were made in another animal and handed over, so passive; and deliberately, so artificial passive.
那些抗体是在另一种动物体内制造后交给他的， 所以是被动；而且是有意的，所以是人工被动。
Four things the examiner keeps asking for.
考官反复要的四点。
First, keep the cells straight.
第一，把细胞分清楚。
Phagocytes engulf anything and are not specific.
吞噬细胞吞噬一切，没有特异性。
B cells make antibodies.
B细胞产生抗体。
T cells are the helpers and the killers.
T细胞是辅助性的和杀伤性的。
Second, when you label an antibody, name the variable region and the antigen-binding site by name — a vague arrow saying "binds here" earns nothing.
第二，标注抗体时，要写出可变区和抗原结合位点的名称—— 画个箭头含糊地写"在这里结合"是拿不到分的。
Third, when you compare the two responses on a graph, say faster and larger, and give memory cells as the reason.
第三，在图上比较两次反应时，要说更快而且更强，并把记忆细胞作为原因。
Fourth, only active immunity makes memory cells, so only active immunity lasts.
第四，只有主动免疫产生记忆细胞，所以只有主动免疫是持久的。
So there it is.
就是这样了。
A first defence that engulfs anything, a second that matches one shape exactly, and a memory that can last a lifetime.
第一道防线吞噬一切，第二道防线精确匹配一种形状， 还有一种可以持续一生的记忆。
Learn these eight terms and the whole topic holds together.
记住这八个术语，整个主题就串起来了。
See you in the next lesson.
下节课见。