Publisher Summary This paper proved to be a work of profound significance to the disciplines of population genetics, transfusion medicine, forensic medicine, and transplantation medicine. Landsteiner was encouraged by his department head to focus more on pathological physiology than anatomy, and this encouragement resulted in his pursuit of an interest in the intraspecies differences of plant and animal proteins, using serological agglutination reactions. The growing appreciation of the profound importance of his work on what was by then known to be the principal human blood groups resulted in this delayed recognition. Apparently, in these cases at least two different sorts of agglutinins are to be distinguished; the first in A, the second in B, and both of them together in C. The cells are indifferent toward agglutinins within the same serum. Moreover, the experiments show that the different sera regarding agglutination do not all work identically. Finally, the chapter mentions that the observations described here allow an explanation of the varying consequences of therapeutic human blood transfusions.