When Austrian physician Karl Landsteiner began investigating blood, science explained the phenomenon where some individual's red blood cells clumped when mixed with other people's blood as the result of some unknown disease or disorder. Landsteiner wasn't convinced and he used his own blood as well as the blood of some colleagues to prove his theory that different people have different types of blood.
Landsteiner used samples to show that humans have different kinds of antigens in the blood. Some of these antigens attacked blood cells harboring other types of antigens. When antigens attack one another, the effect causes clumping of the red blood cells, which in turns leads to the rejection of a blood transfusion and, prior to Landsteiner's research, usually death. In 1901, Landsteiner identified three (and later four) blood types through research on his own blood: A, B, O and AB [source: NobelPrize.org]. Through his self-experimentation, he paved the way for blood type matching that allows for the blood transfusions and organ donation that saves lives today.