Why do blood types matter?

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00:06: It’s often said that despite humanity’s many conflicts,

00:10: we all bleed the same blood.

00:14: It’s a nice thought but not quite accurate.

00:17: In fact, our blood comes in a few different varieties.

00:21: Our red blood cells contain a protein called hemoglobin

00:25: that binds to oxygen,

00:27: allowing the cells to transport it throughout the body.

00:30: But they also have another kind of complex protein

00:33: on the outside of the cell membrane.

00:35: These proteins, known as antigens, communicate with white blood cells,

00:41: immune cells that protect against infection.

00:44: Antigens serve as identifying markers,

00:47: allowing the immune system to recognize your body’s own cells

00:51: without attacking them as foreign bodies.

00:54: The two main kinds of antigens, A and B, determine your blood type.

00:59: But how do we get four blood types from only two antigens?

01:04: Well, the antigens are coded for by three different alleles,

01:08: varieties of a particular gene.

01:11: While the A and B alleles code for A and B antigens,

01:15: the O allele codes for neither,

01:18: and because we inherit one copy of each gene from each parent,

01:22: every individual has two alleles determining blood type.

01:27: When these happen to be different,

01:28: one overrides the other depending on their relative dominance.

01:33: For blood types, the A and B alleles are both dominant, while O is recessive.

01:39: So A and A gives you type A blood, while B and B gives you type B.

01:45: If you inherit one of each,

01:47: the resulting codominance will produce both A and B antigens,

01:52: which is type AB.

01:54: The O allele is recessive,

01:56: so either of the others will override it when they’re paired,

02:00: resulting in either type A or type B.

02:03: But if you happen to inherit two Os, instructions will be expressed

02:07: that make blood cells without the A or the B antigen.

02:12: Because of these interactions,

02:14: knowing both parents’ blood types

02:16: lets us predict the relative probability of their children’s blood types.

02:21: Why do blood types matter?

02:23: For blood transfusions,

02:25: finding the correct one is a matter of life and death.

02:28: If someone with type A blood is given type B blood, or vice versa,

02:33: their antibodies will reject the foreign antigens and attack them,

02:38: potentially causing the transfused blood to clot.

02:42: But because people with type AB blood produce both A and B antigens,

02:46: they don’t make antibodies against them, so they will recognize either as safe,

02:51: making them universal recipients.

02:54: On the other hand,

02:55: people with blood type O do not produce either antigen,

02:58: which makes them universal donors,

03:01: but will cause their immune system to make antibodies

03:04: that reject any other blood type.

03:09: Unfortunately, matching donors and recipients is a bit more complicated

03:13: due to additional antigen systems,

03:16: particular the Rh factor,

03:18: named after the Rhesus monkeys in which it was first isolated.

03:22: Rh+ or Rh- refers to the presence or absence of the D antigen

03:29: of the Rh blood group system.

03:31: And in addition to impeding some blood transfusions,

03:34: it can cause severe complications in pregnancy.

03:38: If an Rh- mother is carrying an Rh+ child,

03:42: her body will produce Rh antibodies that may cross the placenta

03:47: and attack the fetus,

03:49: a condition known as hemolytic disease of the newborn.

03:53: Some cultures believe blood type to be associated with personality,

03:57: though this is not supported by science.

03:58: And though the proportions of different blood types

04:01: vary between human populations,

04:03: scientists aren’t sure why they evolved;

04:06: perhaps as protection against blood born diseases,

04:09: or due to random genetic drift.

04:11: Finally, different species have different sets of antigens.

04:15: In fact, the four main blood types shared by us apes

04:19: seem paltry in comparison to the thirteen types found in dogs.

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