When was the first successful blood transfusion performed between animals and humans?

The earliest recorded animal-to-human blood transfusions were attempted in the 1660s, including work by Jean-Baptiste Denys in France. In 1667, Denys transfused sheep blood into a human patient, but the practice proved dangerous and controversial. These experiments eventually led to restrictions on transfusion for more than a century. Safe transfusion only became possible much later, after blood groups were discovered in 1900.