A transfusion means taking blood from one living thing and injecting it into the bloodstream of another. In the 1660s, scientists experimented with transfusing blood. Richard Lower transfused blood between dogs with some success. Then he injected lamb blood into humans. A patient died and blood transfusion was outlawed.

In 1818, James Blundell transfused blood to a woman from her husband and it worked. But other patients died from transfusions. It was decades later when a German named Leonard Landois learned why blood mixing can be fatal: Sometimes it makes red blood cells clump and explode.

In 1901, American biologist Karl Landsteiner found out why. Some blood has antigens called A and B. (Antigens are special markers on germs and other substances that cause your body to make antibodies. Antibodies are proteins that defend against invaders.) If someone's blood doesn't have A or B, then antigens are harmful to them. Today, blood is tested for antigens before it is transfused.