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In theory, the process is simple.
从理论上讲，这个过程很简单。
People give some blood.
人们献血。
Health workers then store the blood under controlled conditions.
然后，医护人员将血液储存在可控的环境中。
It is then ready for use when the doctors need it.
当医生需要血液时，就可以使用了。
But in real life, it is not that easy.
但在现实生活中，这并不容易。
Doctors cannot just give any kind of blood to their patients.
医生不能随便给他们的病人输血。
The reason for this is genetic.
遗传因素造成了这种情况。
The genes passed on to us from our parents dictate what our blood group will be.
父母遗传给我们的基因决定了我们的血型。
If a person receives blood from the wrong blood group, it could have a bad effect on their health.
如果一个人接受了错误血型的血液，可能会对他们的健康产生不良影响。
It may even result in death.
它甚至可能会导致死亡。
Health workers must supply patients with blood that fits their blood group.
医护人员必须为患者提供符合他们血型的血液。
There are four kinds of blood group: O, A, B and AB.
有四种血型：O型、A型、B型和AB型。
The most common blood kind, or type, is O.
最常见的血型是O型。
Any patient needing fresh blood can receive type O.
任何需要新鲜血液的患者都可以输O型血。
Scientists believe that type O is the oldest blood group.
科学家认为O型血是最古老的血型。
They say that A and B groups developed from the O blood type as human history developed.
他们说，随着人类历史的发展，A型血和B型血是从O型血发展而来的。
Later, people moved around the world.
后来，人们在世界各地活动。
They often married individuals with a very different genetic history.
他们经常与有着不同基因历史的人结婚。
The scientists believe that group AB formed by genetic mixing among people with other types.
科学家们认为，AB型血是由其他血型的人基因混合形成的。
Type A blood contains a particular kind of sugar molecule.
A型血含有一种特殊的糖分子。
This sugar molecule attaches itself to the surface of the red blood cells.
这种糖分子附着在红细胞表面。
And it is an antigen - it can make the body react as if it is being attacked by harmful bacteria.
它是抗原——它可以使身体做出反应，就好像受到了有害细菌的攻击一样。
This is what would happen if doctors gave B or AB type blood to someone with blood type A.
如果医生给A型血的人输B型或AB型血，就会发生这种情况。
