Diseases and Immunity
IGCSE Biology Topic 10 6:05 English narration · English + 中文 subtitles burned in
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Right now, your body is being tested by microbes — on your skin, in the air you breathe, in the food you eat.
此时此刻，你的身体正在被微生物考验——在你的皮肤上、你呼吸的空气里、你吃的食物中。
Almost all of them never get in.
它们几乎全都进不来。
And when one does, your body does something remarkable: it learns.
而当有一个进来时，你的身体会做一件了不起的事：它会学习。
Meet the same pathogen twice, and the second time you may not even notice.
同一种病原体遇到第二次，你可能根本察觉不到。
Welcome to diseases and immunity.
欢迎来到疾病与免疫。
Pathogens, the body's defences, vaccination, and cholera.
病原体、身体的防御、疫苗接种，还有霍乱。
Let's begin.
让我们开始吧。
Start with two definitions, because both are worth marks.
先从两个定义开始，两个都值分。
A pathogen is an organism that causes disease.
病原体是引起疾病的生物。
A transmissible disease is one where the pathogen can pass from one host to another.
传染病是病原体能从一个宿主传到另一个宿主的疾病。
Notice the word host — that is the organism the pathogen lives in.
注意"宿主"这个词—— 就是病原体寄居的那个生物。
A pathogen is transmitted in two main ways.
病原体主要通过两种方式传播。
Directly, by contact — for example through blood or other body fluids.
直接传播，通过接触——比如通过血液或其他体液。
Or indirectly, from contaminated surfaces, food, water, animals, or the air.
或者间接传播，通过被污染的表面、食物、水、动物或空气。
If a question asks how a disease spreads, say which of the two, and give an example.
如果题目问一种疾病如何传播，就说是哪一种，并举一个例子。
Your body has five defences you need to name, and there is a pattern: four of them keep pathogens out, and one deals with those that get in.
你的身体有五道防御需要你说出名字，而且有一个规律：四道把病原体挡在外面， 一道处理进来的。
First, the skin is a barrier that covers and protects the whole body.
第一，皮肤是覆盖并保护全身的屏障。
Second, hairs in your nose trap dust and pathogens in the air you breathe in.
第二，鼻子里的毛拦住你吸入空气中的灰尘和病原体。
Third, mucus, the sticky liquid in your airways, traps pathogens.
第三，黏液，也就是呼吸道里黏黏的液体，粘住病原体。
Fourth, stomach acid kills most pathogens in your food.
第四，胃酸杀死食物中大部分病原体。
And fifth, if anything does get inside, white blood cells find and destroy it.
第五，如果真有东西进去了，白细胞会找到并摧毁它。
Four barriers, one hunter.
四道屏障，一个猎手。
Beyond your own body, public health stops disease spreading through a whole population.
除了你自己的身体，公共卫生还能阻止疾病在整个人群中传播。
Three measures, and each one blocks a route we just met.
三项措施，每一项都堵住我们刚说的一条传播途径。
A clean water supply, so drinking water carries no pathogens.
清洁的水源，让饮用水不带病原体。
Hygienic food preparation and good personal hygiene, such as washing your hands.
卫生的食物制备和良好的个人卫生，比如洗手。
And proper waste disposal and sewage treatment, so waste does not contaminate water or food.
还有妥善的垃圾处理和污水处理，让废物不污染水或食物。
Each measure cuts off indirect transmission at a different point.
每一项措施都在不同的地方切断间接传播。
Now the supplement material, and it reuses an idea you already know.
现在是补充部分的内容，它用到你已经知道的一个概念。
Every pathogen carries antigens on its surface, and each kind of antigen has its own special shape.
每一个病原体表面都带有抗原，每一种抗原都有自己特殊的形状。
Antibodies are proteins made by lymphocytes, a kind of white blood cell.
抗体是由淋巴细胞制造的蛋白质，淋巴细胞是一种白细胞。
And here is the key sentence: an antibody has a shape that is complementary to one antigen, so it fits only that antigen.
关键的一句话是：抗体的形状与某一种抗原互补，所以它只吻合那一种抗原。
That is exactly the lock-and-key idea from enzymes, used again.
这正是酶那里的锁和钥匙概念，再次被用到。
When antibodies bind to a pathogen, two things happen: they either destroy the pathogen directly, or they mark it so that phagocytes destroy it.
当抗体结合到病原体上时，会发生两件事：要么直接摧毁病原体， 要么给它做标记，让吞噬细胞去摧毁它。
Active immunity is protection made by your own body producing antibodies.
主动免疫是你自己的身体产生抗体所形成的保护。
You get it in two ways: after an actual infection, or after vaccination.
你有两种方式获得它： 在真正感染之后，或者在接种疫苗之后。
Learn how vaccination works in three steps.
学会疫苗接种的三个步骤。
One: weakened pathogens, or just their antigens, are put into the body — they cannot make you ill.
第一：把减弱的病原体，或者只是它们的抗原，放进身体——它们不会让你生病。
Two: those antigens make your lymphocytes produce antibodies.
第二：这些抗原让你的淋巴细胞产生抗体。
Three, and this is the step that matters most: your body also makes memory cells that stay for years.
第三，这是最重要的一步：你的身体还会制造能存活多年的记忆细胞。
So if the real pathogen arrives later, the memory cells make antibodies very fast, and you do not become ill.
所以如果真正的病原体后来到来，记忆细胞会很快制造抗体，你就不会生病。
And if most people in a group are vaccinated, the disease cannot spread easily even to those who are not.
而且如果一个群体中大多数人接种了疫苗，疾病就很难传播， 即使传给那些没接种的人也很难。
Here is the graph question.
这是图表题。
After a first dose of a vaccine, antibody levels rise slowly and stay low.
第一剂疫苗之后，抗体水平上升缓慢，而且一直很低。
After a second dose of the same vaccine, they rise faster and reach a much higher level.
同一种疫苗的第二剂之后，抗体上升更快，达到高得多的水平。
Explain why.
解释为什么。
At the first dose the body has no memory cells for that antigen, so time is lost while the right lymphocyte is found and multiplies — the response is slow and small.
第一剂时身体对那种抗原还没有记忆细胞，所以要花时间找到合适的淋巴细胞并让它增殖—— 反应又慢又小。
But the second dose meets the memory cells left over from the first.
但第二剂遇到的是第一剂留下的记忆细胞。
They recognise the antigen at once, so antibodies are made sooner and in far greater numbers.
它们立刻认出抗原，所以抗体产生得更早，数量也多得多。
The marks are for memory cells.
得分点是记忆细胞。
Writing that the body is used to it earns nothing.
写"身体已经习惯了"什么分也得不到。
Passive immunity is different.
被动免疫不一样。
It is protection from antibodies made by another body, not your own.
它是由另一个身体制造的抗体提供的保护，不是你自己制造的。
The example you must know is a baby: it receives antibodies from its mother across the placenta before birth, and in breast milk after birth.
你必须知道的例子是婴儿：出生前它通过胎盘从母亲那里获得抗体， 出生后通过母乳获得。
That is why breast-feeding helps protect infants.
这就是为什么母乳喂养有助于保护婴儿。
But passive immunity is only short-term, and the reason is the whole point: the baby did not make those antibodies itself, so no memory cells are made.
但被动免疫只是短期的，原因正是关键所在： 婴儿自己没有制造那些抗体，所以没有产生记忆细胞。
No memory cells, no long-term protection.
没有记忆细胞，就没有长期保护。
Last, for the supplement paper: cholera.
最后是补充部分：霍乱。
Cholera is caused by a bacterium, and it spreads in contaminated water — which links it straight back to a clean water supply.
霍乱由一种细菌引起，它通过被污染的水传播—— 这正好和清洁水源联系起来。
Learn the chain in order.
按顺序记住这条链。
The bacterium makes a toxin.
细菌产生一种毒素。
That toxin causes chloride ions to be pumped into the small intestine.
这种毒素使氯离子被泵入小肠。
Because there are now more ions in the intestine, water moves into it by osmosis.
因为小肠里现在离子更多了， 水就通过渗透作用移进去。
And the result is diarrhoea and dehydration, together with a loss of ions from the blood.
结果就是腹泻和脱水，同时血液中的离子也流失。
Notice the osmosis step — the examiner wants it named.
注意渗透这一步——考官要你说出它的名字。
Three marks to lock in.
锁住三个分。
First: name all five defences — skin, nose hairs, mucus, stomach acid, and white blood cells.
第一：说出全部五道防御——皮肤、鼻毛、黏液、胃酸，还有白细胞。
Second: active makes memory cells, so it lasts; passive gives you someone else's antibodies and makes none, so it is short-term.
第二：主动免疫产生记忆细胞，所以持久；被动免疫给你的是别人的抗体， 不产生记忆细胞，所以是短期的。
Third: in cholera, name osmosis — the toxin pumps chloride ions into the gut, and water follows by osmosis.
第三：霍乱题里要说出渗透——毒素把氯离子泵进肠道，水随之通过渗透进入。
Master these, and this topic is yours.
掌握这些，这个专题就是你的了。