Question: When did blood transfusions become safer and why?
Disambiguated questions and answers: question: What year did blood transfusions become safer thanks to Karl Landsteiner?
With what event did blood transfusions become safer and why in 1901?
What year did blood transfusions become safer thanks to Rhesus blood system discovery?
With what ?event did blood transfusions become safer and why in 1937-1940?
answer: 1901
Karl Landsteiner discovered three human blood groups (O, A, and B)
1937–40
discovered the Rhesus blood group system, which was found to be the cause of the majority of transfusion reactions
Natural Questions answer: 1901 (from the Wikipedia article "Blood transfusion")