What year did blood transfusions become safe?

Blood transfusions became relatively safe in the 1900s with the discovery of blood types and cross-matching. The discovery of blood groups (A, B, AB, and O) by Karl Landsteiner in 1900 laid the foundation for safe blood transfusions, allowing doctors to match the blood of donors and recipients based on compatibility, reducing the risk of adverse reactions. Further advancements, like compatibility testing and the development of blood storage techniques in the 20th century, have significantly improved the safety and efficacy of blood transfusions.