we all bleed the same blood.

我們流的血都是一樣的

It's a nice thought but not quite accurate.

這想法雖佳，不過不太正確

In fact, our blood comes in a few different varieties.

其實，我們的血液本自有一些不同差異

Our red blood cells contain a protein called hemoglobin

我們的紅血球含有一種叫做血紅蛋白的蛋白質--

that binds to oxygen,

--可以和氧結合

allowing the cells to transport it throughout the body.

讓紅血球能夠將之運送到全身

But they also have another kind of complex protein

可是它們還有另外一種複合性蛋白質

on the outside of the cell membrane.

在細胞膜的外面

These proteins, known as antigens, communicate with white blood cells,

這一些蛋白質 -- 稱為抗原 -- 跟白血球溝通

immune cells that protect against infection.

[白血球是] 對抗感染的免疫細胞

Antigens serve as identifying markers,

抗原作為辨識標記

allowing the immune system to recognize your body's own cells

讓免疫系統可以辨識你自己身體的細胞

without attacking them as foreign bodies.

而不會當作異物攻擊它們

The two main kinds of antigens, A and B, determine your blood type.

兩種主要的抗原 -- A 和 B -- 決定你的血型

But how do we get four blood types from only two antigens?

不過我們如何只從兩種抗原而得到四種血型呢？

Well, the antigens are coded for by three different alleles,

是這樣的，這些抗原是依三種不同的 “對偶基因” 來編碼

varieties of a particular gene.

[對偶基因] 是一種特殊基因的種類

While the A and B alleles code for A and B antigens,

A 和 B 對偶基因編碼 A 型和 B 型抗原，

the O allele codes for neither,

而 O 對偶基因則沒有編碼任何一種

and because we inherit one copy of each gene from each parent,

而且因為我們從父母各自遺傳一組基因，

every individual has two alleles determining blood type.

每人擁有兩個決定血型的對偶基因

When these happen to be different,

當這些編碼不同，

one overrides the other depending on their relative dominance.

依據他們的相對顯性，一個會覆蓋另一個

For blood types, the A and B alleles are both dominant, while O is recessive.

就血型來說，A 和 B 對偶基因都是顯性，而 O 是隱性

So A and A gives you type A blood, while B and B gives you type B.

所以 A 和 A 結合就成為 A 型，而 B 和 B 結合就成為 B 型

If you inherit one of each,

如果你遺傳了其中各一個，

the resulting codominance will produce both A and B antigens,

所產生的共顯性會製造 A 與 B 抗原

which is type AB.

這就是 AB 型

The O allele is recessive,

O 對偶基因是隱性的

so either of the others will override it when they're paired,

所以其他任何一個和它配對都會覆蓋它

resulting in either type A or type B.

形成不是A 型就是 B 型

But if you happen to inherit two Os, instructions will be expressed

可是如果你剛好遺傳到兩個 O，指令將下達

that make blood cells without the A or the B antigen.

製造沒有 A 或 B 抗原的血球

Because of these interactions,

因為這些交互作用，

knowing both parents' blood types

知道雙親的血型，

lets us predict the relative probability of their children's blood types.

就可以讓我們預測他們的小孩血型的相對機率

Why do blood types matter?

血型為什麼重要呢？

For blood transfusions,

就輸血而言，

finding the correct one is a matter of life and death.

找對正確的血是攸關生死的

If someone with type A blood is given type B blood, or vice versa,

如果一個 A 型的人輸了 B 型的血，反之亦然，

their antibodies will reject the foreign antigens and attack them,

他們的抗體就會排斥外來的抗原並攻擊它們

potentially causing the transfused blood to clot.

可能會導致所輸入的血凝固

But because people with type AB blood produce both A and B antigens,

不過因為 AB 型的人會製造出 A 抗原及 B 抗原，

they don't make antibodies against them, so they will recognize either as safe,

他們不會製造抗體來抵抗它們，所以將任何血型都視為安全

making them universal recipients.

讓他們成為 “全適受血者”

On the other hand,

反過來說，

people with blood type O do not produce either antigen,

O 型的人不製造任何一種抗原

which makes them universal donors,

使他們成為 “全適捐血者”

but will cause their immune system to make

不過他們的免疫系統會製造

antibodies that reject any other blood type.

排斥其他任何血型的抗體

Unfortunately, matching donors and recipients is a bit more complicated

不幸的是，要配對捐血者和受血者不是那麼簡單的事情

due to additional antigen systems,

因為還要將其他的抗原系統考慮在內

particular the Rh factor,

尤其是 Rh 因子

named after the Rhesus monkeys in which it was first isolated.

-- 以第一次在恆河猴身上分離出來而得名

Rh+ or Rh- refers to the presence or absence of the D antigen of the Rh blood group system.

Rh 陽性或 Rh 陰性意指 Rh 血型系統裡是否存在 D 抗原

And in addition to impeding some blood transfusions,

除了妨礙一些輸血之外

it can cause severe complications in pregnancy.

它也可能導致懷孕時的嚴重併發症

If an Rh- mother is carrying an Rh+ child,

如果一位 Rh 陰性的母親懷有 Rh 陽性的孩子

her body will produce Rh antibodies that may cross the placenta

她的身體就會製造 Rh 的抗體，可能穿過胎盤

and attack the fetus,

攻擊胎兒

a condition known as hemolytic disease of the newborn.

這種情況叫做 “新生兒溶血症”

Some cultures believe blood type to be associated with personality,

有些文化相信血型和個性是有關的

though this is not supported by science.

儘管科學不支持這種說法

And though the proportions of different blood types

雖然不同血型的分佈

vary between human populations,

在各種人種之間是有差異的

scientists aren't sure why they evolved;

科學家不確定為什麼會有如此的演化

perhaps as protection against blood born diseases,

也許是為了對抗血液衍生的疾病

or due to random genetic drift.

或者是因隨機的遺傳漂變

Finally, different species have different sets of antigens.

最後，不同物種有不同的抗原組合

In fact, the four main blood types shared by us apes

其實，我們人猿的四種主要血型

seem paltry in comparison to the thirteen types found in dogs.

相較於狗類的十三種，看起來是微不足道的