His third group (C) indicated absence of both A and B antigens, but contains anti-A and anti-B. The following year, his students Adriano Sturli and Alfred von Decastello discovered the fourth type (but not naming it, and simply referred to it as "no particular type"). In 1910, Ludwik Hirszfeld and Emil Freiherr von Dungern introduced the term O (null) for the group Landsteiner designated as C, and AB for the type discovered by Sturli and von Decastello. They were also the first to explain the genetic inheritance of the blood groups. Classification systems
Czech serologist Jan Janský independently introduced blood type classification in 1907 in a local journal. He used the Roman numerical I, II, III, and IV (corresponding to modern O, A, B, and AB). Unknown to Janský, an American physician William L. Moss devised a slightly different classification using the same numerical; his I, II, III, and IV corresponding to modern AB, A, B, and O.These two systems created confusion and potential danger in medical practice. Mosss system was adopted in Britain, France, and US, while Janskýs was preferred in most European countries and some parts of US. To resolve the chaos, the American Association of Immunologists, the Society of American Bacteriologists, and the Association of Pathologists and Bacteriologists made a joint recommendation in 1921 that the Jansky classification be adopted based on priority. But it was not followed particularly where Mosss system had been used.In 1927, Landsteiner had moved to the Rockefeller Institute for Medical Research in New York.
It remains to be seen whether this appearance is related to inborn differences between individuals or it is the result of some damage of bacterial kind. This was the first evidence that blood variations exist in humans – it was believed that all humans have similar blood. The next year, in 1901, he made a definitive observation that blood serum of an individual would agglutinate with only those of certain individuals. Based on this he classified human blood into three groups, namely group A, group B, and group C. He defined that group A blood agglutinates with group B, but never with its own type. Similarly, group B blood agglutinates with group A. Group C blood is different in that it agglutinates with both A and B.This was the discovery of blood groups for which Landsteiner was awarded the Nobel Prize in Physiology or Medicine in 1930. In his paper, he referred to the specific blood group interactions as isoagglutination, and also introduced the concept of agglutinins (antibodies), which is the actual basis of antigen-antibody reaction in the ABO system. He asserted:

[It] may be said that there exist at least two different types of agglutinins, one in A, another one in B, and both together in C. The red blood cells are inert to the agglutinins which are present in the same serum. Thus, he discovered two antigens (agglutinogens A and B) and two antibodies (agglutinins – anti-A and anti-B).