It's often said that despite humanity's many conflicts, we all bleed the same blood.
人们常说，除去人性中的诸多冲突，我们都流着同样的血。
It's a nice thought but not quite accurate.
这种想法很好，但并不完全准确。
In fact, our blood comes in a few different varieties.
事实上，我们的血液由几种成分构成。
Our red blood cells contain a protein called hemoglobin that binds to oxygen,
我们的红细胞包含一种能携带氧气的称为血红蛋白的的蛋白质，
allowing the cells to transport it throughout the body.
它能让细胞将氧气输送到全身。
But they also have another kind of complex protein on the outside of the cell membrane.
但在细胞膜外，它们拥有另一种复杂的蛋白质。
These proteins, known as antigens, communicate with white blood cells, immune cells that protect against infection.
这些蛋白质，如抗原负责与白细胞进行交流，如免疫细胞则负责抵御感染。
Antigens serve as identifying markers,
抗原充当着识别标记者的身份，
allowing the immune system to recognize your body's own cells without attacking them as foreign bodies.
它让免疫系统识别出你体内自己的细胞，而不会将他们当作外来的物质进行攻击。
The two main kinds of antigens, A and B, determine your blood type.
这两种主要的抗原A和B决定了你的血型。
But how do we get four blood types from only two antigens?
但仅从这两种抗原我们怎么得到四种血型的呢？
Well, the antigens are coded for by three different alleles, varieties of a particular gene.
这些抗原被三种不同的等位基因进行编码，成为特定基因的种群。
While the A and B alleles code for A and B antigens, the O allele codes for neither,
A和B基因进行编码成为A和B抗原，O基因则一个都不是，
and because we inherit one copy of each gene from each parent, every individual has two alleles determining blood type.
因为我们从每一个父母那继承了每个基因的复制品，每个人拥有两个决定血型的等位基因。
When these happen to be different, one overrides the other depending on their relative dominance.
当这些基因出现不同时，根据它们间相对的主导地位其中一个会覆盖另一个。
For blood types, the A and B alleles are both dominant, while O is recessive.
对于血型来说，A和B都是显性的，O则是隐性的。
So A and A gives you type A blood, while B and B gives you type B.
所以A和A组合产生A型血，B和B组合产生B型血。
If you inherit one of each, the resulting codominance will produce both A and B antigens, which is type AB.
如果你每一种基因继承了一个，根据共显性会同时生产出A和B抗原，这就是AB型血。
The O allele is recessive, so either of the others will override it when they're paired, resulting in either type A or type B.
O基因是隐性的，所以当它和其他基因配对时都将被覆盖，成为A型血或B型血。

But if you happen to inherit two Os, instructions will be expressed that make blood cells without the A or the B antigen.
但如果你继承了两个O基因时，将会产生出没有A和B抗原的血球。
Because of these interactions, knowing both parents' blood types lets us predict the relative probability of their children's blood types.
因为这些基因间的相互作用，了解父母双方的血型能让我们预测他们孩子血型的相对概率。
Why do blood types matter?
为什么血型那么重要呢？
For blood transfusions, finding the correct one is a matter of life and death.
对于输血来说，找对血型是关乎生死的大事。
If someone with type A blood is given type B blood, or vice versa,
如果给一个A型血的人输了B型血，或者反过来，
their antibodies will reject the foreign antigens and attack them, potentially causing the transfused blood to clot.
他们的抗体会拒绝外来抗原并攻击它们，将可能导致输入的血凝块。
But because people with type AB blood produce both A and B antigens, they don't make antibodies against them,
但由于AB型血的人生产A和B两种抗原，他们对A和B型血都不产生抗体，
so they will recognize either as safe, making them universal recipients.
所以他们对于两种血都能安全接受，这使得AB型血的人“来者不拒”。
On the other hand, people with blood type O do not produce either antigen, which makes them universal donors,
另一方面，O型血的人不生产任何一种抗原，这就使得他们成为了大众捐献者，
but will cause their immune system to make antibodies that reject any other blood type.
但这也会导致他们的免疫系统制造抗体来拒绝其他任何一种血型。
Unfortunately, matching donors and recipients is a bit more complicated due to additional antigen systems,
不幸的是，由于额外的抗原系统，将捐献者与接收者匹配稍微有点复杂，
particular the Rh factor, named after the Rhesus monkeys in which it was first isolated.
尤其是Rh因子，这个因子是以恒河猴第一次被隔离之后的地方命名的。
Rh+ or Rh- refers to the presence or absence of the D antigen of the Rh blood group system.
Rh+或Rh-指的是在Rh血群系统中D抗原的存在或缺少。
And in addition to impeding some blood transfusions, it can cause severe complications in pregnancy.
除了阻碍一些输血外，它还会在怀孕过程中产生严重的并发症。
If an Rh- mother is carrying an Rh+ child, her body will produce Rh antibodies that may cross the placenta and attack the fetus,
如果一个Rh-型的妈妈怀着一个Rh+型的宝宝，她的体内会产生Rh抗体，这种抗体会通过胎盘攻击胎儿。
a condition known as hemolytic disease of the newborn.
这就是大家所熟知的新生儿溶血病。
Some cultures believe blood type to be associated with personality, though this is not supported by science.
一些文化认为血型与一个人的个性密切相关，虽然这并未经科学证实。
And though the proportions of different blood types vary between human populations, scientists aren't sure why they evolved;
虽然不同血型的比例在人群中大不相同，科学家也不能确定为什么它们会这样发展；
perhaps as protection against blood born diseases, or due to random genetic drift.
也许是为了防止血液传播疾病，或者只是任意的基因漂流。
Finally, different species have different sets of antigens.
最终，不同的种群有不同的抗体。
In fact, the four main blood types shared by us apes seem paltry in comparison to the thirteen types found in dogs.
事实上，我们人类和猿所拥有的四种血型，与狗的十三种血型相比简直微不足道。