On March 29, Karl Landsteiner published the findings that would form the basis of the ABO blood group classification, a discovery that clarified why blood transfusions sometimes succeeded and sometimes caused fatal reactions. Working at the Wilhelminenspital in Vienna and later associated with the University of Vienna, Landsteiner had been investigating the agglutination (clumping) of red blood cells when mixed with serum from different individuals. In his experiments he observed consistent patterns of reaction that allowed him to divide human blood into distinct groups, which he labeled A, B and C (the latter later renamed O). These groupings explained reciprocal agglutination reactions and provided a framework for matching donor and recipient blood to avoid dangerous incompatibilities. Landsteiner’s work was grounded in careful laboratory observation rather than theoretical speculation. He reported that sera from some individuals would agglutinate the red cells of others but not vice versa, showing that individual variation in the properties of red blood cells and serum antibodies accounted for transfusion outcomes. By identifying reproducible categories, his classification transformed the clinical approach to transfusion medicine: matching blood groups could prevent immediate hemolytic reactions and save lives. The original publication and related communications in the early 1900s were not instant catalysts for universal change; adoption of blood-group testing and transfusion protocols proceeded over subsequent years as other clinicians and researchers validated and extended his findings. Landsteiner’s discovery also prompted further immunological and biochemical research that led to expansion of blood-group systems beyond ABO and to an improved understanding of antibodies and antigens on red-cell surfaces. Landsteiner received the Nobel Prize in Physiology or Medicine in 1930 for his contributions to immunology and blood chemistry, recognition that explicitly cited the significance of his blood-group work. The ABO classification remains a foundational element of transfusion practice and has had enduring impacts on surgery, emergency medicine, obstetrics and hematology. It also enabled the later development of blood banks and large-scale transfusion services, which depended on reliable typing and crossmatching to maintain safety. Historical notes: contemporary accounts sometimes vary on exact dating of specific reports and press communications; Landsteiner’s principal papers describing the grouping were published in the first decade of the 20th century. Secondary literature and archival sources confirm his central role in discovering and promoting the ABO classification but place his work in the context of incremental advances by other clinicians and laboratory scientists in serology and transfusion technique.