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  • It's often said that despite humanity's many conflicts,

  • we all bleed the same blood.

  • It's a nice thought but not quite accurate.

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

  • Our red blood cells contain a protein called hemoglobin

  • that binds to oxygen,

  • allowing the cells to transport it throughout the body.

  • But they also have another kind of complex protein

  • on the outside of the cell membrane.

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

  • immune cells that protect against infection.

  • Antigens serve as identifying markers,

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

  • without attacking them as foreign bodies.

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

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

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

  • varieties of a particular gene.

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

  • the O allele codes for neither,

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

  • every individual has two alleles determining blood type.

  • When these happen to be different,

  • one overrides the other depending on their relative dominance.

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

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

  • If you inherit one of each,

  • the resulting codominance will produce both A and B antigens,

  • which is type AB.

  • 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.

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

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

  • Because of these interactions,

  • knowing both parents' blood types

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

  • Why do blood types matter?

  • For blood transfusions,

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

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

  • their antibodies will reject the foreign antigens and attack them,

  • potentially causing the transfused blood to clot.

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

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

  • making them universal recipients.

  • On the other hand,

  • people with blood type O do not produce either antigen,

  • which makes them universal donors,

  • but will cause their immune system to make

  • antibodies that reject any other blood type.

  • Unfortunately, matching donors and recipients is a bit more complicated

  • due to additional antigen systems,

  • particular the Rh factor,

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

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

  • And in addition to impeding some blood transfusions,

  • it can cause severe complications in pregnancy.

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

  • her body will produce Rh antibodies that may cross the placenta

  • and attack the fetus,

  • a condition known as hemolytic disease of the newborn.

  • Some cultures believe blood type to be associated with personality,

  • though this is not supported by science.

  • And though the proportions of different blood types

  • vary between human populations,

  • scientists aren't sure why they evolved;

  • perhaps as protection against blood born diseases,

  • or due to random genetic drift.

  • Finally, different species have different sets of antigens.

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

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

It's often said that despite humanity's many conflicts,

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B1 US TED-Ed blood blood type antigen inherit recessive

【TED-Ed】Why do blood types matter? - Natalie S. Hodge

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    Ann posted on 2015/08/04
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