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An Australian man named James Harrison holds the world record for most blood donations. His blood has saved the lives of millions of newborn babies, but how can one man's blood help babies all over the world?

Hosted by: Hank Green
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Hank: The world record holder for making the most blood donations is an Australian man named James Harrison. He's made more than 1,000 donations in the past 60 years, and is credited with saving over two million lives, mainly newborn babies. So how can a stranger's blood save a baby's life?

Sometimes, a fetus can have a different blood type than its mother, and when that happens, it can spell trouble. Your red blood cells have proteins called antigens on their surfaces and they vary depending on your genetics. You've probably heard of type A and type B blood, for example. If you have type A, that means your red blood cells have the A antigens.

And the antigen that causes trouble in pregnancy is the D antigen, also known as the Rh factor. If you have the D antigen, then you're Rh-positive. If you don't, you're Rh-negative. And when a pregnant woman is Rh-negative, that's when there's a risk of a problem, because if their D antigens aren't the same, the mother's immune system treats the fetus's blood cells as foreign invaders and attacks them, which can lead to anemia in the fetus and sometimes even heart failure.

But the first time a woman becomes pregnant with an Rh-positive fetus, this isn't much of an issue because she hasn't developed a strong immune response to the Rh factor so her immune system won't attack enough of the fetus's red blood cells to be dangerous. After that, though, she'll be sensitized to the Rh-positive red blood cells, so if she becomes pregnant again and the fetus is positive again, that can cause a condition called hemolytic disease of the newborn, which can be a big problem.

The mother's immune system will mount an attack, killing many of the fetus's red blood cells, causing anemia or a deficiency in red blood cells. If the fetus doesn't have enough red blood cells, its heart has to work harder to deliver oxygen to the rest of its body. And in some cases, that can lead to heart failure.

Meanwhile, all those destroyed red blood cells are getting broken down by a compound called bilirubin. Bilirubin is processed by the liver, but often the fetus's liver can't handle the overload. That causes jaundice, which can lead to brain damage when it's severe. But blood like Harrison's can stop all of this from happening because it contains high levels of a certain antibody called anti-D immunoglobulin.

When anti-D is injected into a pregnant woman, it can stop her immune system from attacking the fetus's blood cells. The antibodies destroy any Rh-positive blood cells from the fetus that end up in the mother's blood stream before she develops an immune response to them. But the anti-D doesn't pass through to the fetus, so the red blood cells inside the fetus remain safe.

Anti-D injections come from the blood plasma of people with unusually high levels of the anti-D immunoglobulin antibody in their blood, like James Harrison. They're a standard treatment for Rh-negative pregnant women at around 28 weeks of gestation before too much of the fetus's blood has passed into the mother's blood stream.

Usually, doctors don't know for sure whether the fetus is positive or negative, so all Rh-negative pregnant women get treated just in case. If they get pregnant again, it might just save the baby's life.

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Michael: For a while, lots of people thought this mild, pleasant scent was just baby powder or sweet-smelling wipes. Others claimed it was just a myth, a hallucination by sleep-deprived new parents. But, just like (audio fades out).