Answer
Transfusion reactions, once a common and often fatal complication of blood transfusions, are now largely preventable thanks to Landsteiner's identification of agglutinins. Agglutinins are antibodies that cause clumping of red blood cells when incompatible blood types are mixed. By determining blood type compatibility before transfusions, doctors can avoid transfusing patients with incompatible blood, thus preventing potentially life-threatening reactions.