How did Karl Landsteiner’s discovery of the ABO blood groups improve blood transfusions?

Karl Landsteiner’s 1901 discovery of the ABO blood group system transformed blood transfusion from a dangerous, often fatal practice into a reliable, lifesaving medical procedure.12

Prior to his work, the underlying cause of transfusion-related deaths was unknown, though it was observed that mixing blood from different individuals could cause the clumping, or agglutination, of red blood cells. Landsteiner identified that this reaction is an immunological response occurring when a recipient’s immune system produces antibodies against foreign antigens on the surface of donor red blood cells.1

By categorizing blood into groups based on these antigens, Landsteiner made it possible to match donor and recipient blood types accurately. This compatibility testing prevents the host's immune system from attacking the donated blood, thereby avoiding hemolytic transfusion reactions that can lead to death. His discovery established the foundation for modern transfusion medicine and remains a vital safety standard in clinical practice today.123