James Harrison, a man from Australia known as "The Man with the Golden Arm," possesses a rare blood type that has saved the lives of two million babies from Rhesus (Rh) disease, a condition that can cause heart failure in a fetus if not treated.

In SciShow's video, "How Can One Person's Blood Save 2 Million Babies?" host Hank Green explains Harrison's blood has the remarkable ability to save countless lives because it contains high levels of the antibody anti-D immunoglobulin. When anti-D is injected into an Rh-negative pregnant woman, it can stop her immune system from attacking her fetus' blood cells.

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So, what exactly makes Rh-negative dangerous for newborns?

A fetus can have a different blood type than its mother, which can lead to an array of health issues. All red blood cells have proteins called antigens, and they vary depending on genetics. The d antigen, known as the "Rh factor," causes trouble in pregnancy when a pregnant woman is Rh-negative. If a mother and her fetus' d antigens aren't the same, the mother's immune system treats the fetus' blood cells as foreign invaders, and attacks them, which can lead to anemia in the fetus, and sometimes heart failure.

Initially, a woman who becomes pregnant with an Rh-positive fetus won't face too many complications because she hasn't developed a strong immune response to the RH factor, which means her immune system won't attack enough of the fetus' red blood cells to be dangerous. After that, however, she'll be sensitized to Rh-positive red blood cells.

If she becomes pregnant again and the fetus is positive, this can cause haemolytic disease, a condition that can lead to the killing of the fetus's red blood cells, and anemia. 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 this can lead to heart failure.

Meanwhile, all those destroyed red blood cells are getting down broken by a compound called bilirubin — it's processed by the liver, but often the fetus' liver can't handle the overload, and this causes jaundice, which can lead to brain damage when it's severe.

Harrison's blood can stop the progression of Rh disease because of anti-D. The antibodies destroy any Rh-positive blood cells from the fetus that end up in the mother's bloodstream before she develops an immune response to them. The anti-D doesn't pass through to the fetus, so the red blood cells inside the fetus remain safe. Anti-D injections are a standard treatment for Rh-negative pregnant women at around 28 weeks of gestation, before too much of the fetus' blood has passed into the mother's bloodstream.

Doctors still aren't exactly sure why Harrison has this rare blood type, but they think it may be linked to he transfusions he received when he was 14 after a lung surgery, CNN reported. In 1951, when Harrison was 14, he had a chest operation where they removed a lung. He received 13 liters of blood.

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Decades later, Harrison's blood has saved the lives of millions of babies all over the world.

See Also:

7 Little-Known Facts About Blood

How Does A Blood Transfusion Change Your Body And DNA?