The first person to successfully perform a blood transfusion is often credited to the French physician Jean-Baptiste Denis in the 17th century. In 1667, he transfused sheep's blood into a young man, marking one of the earliest documented instances of animal-to-human transfusion. The procedure garnered significant attention, as it led to both successes and failures, raising ethical and scientific questions about blood types and compatibility. While Denis's work laid foundational concepts for transfusion medicine, the practice advanced significantly in the 19th and 20th centuries with the discovery of blood groups and improved techniques, becoming a critical component of modern medicine.