In 1881, doctor William Halsted rushed to help his sister Minnie, who was hemorrhaging after childbirth.
He quickly inserted a needle into his arm, withdrew his own blood, and transferred it to her.
After a few uncertain minutes, she began to recover.
Halsted didn't know how lucky they'd gotten.
His transfusion only worked because he and his sister happened to have the same blood type — something that isn't guaranteed, even among close relatives.
Blood types hadn't been discovered by Halsted's time, though people had been experimenting with transfusions for centuries — mostly unsuccessfully.
In 1667, a French physician named Jean-Baptiste Denis became the first to try the technique on a human.
Denis transfused sheep's blood into Antoine Mauroy, a man likely suffering from psychosis, in the hopes that it would reduce his symptoms.
Afterward, Mauroy was in good spirits.
But after a second transfusion, he developed a fever, severe pain in his lower back, intense burning in his arm, and he urinated a thick, black liquid.