Blood type transfusion can be considered safe as long as the serum of the recipient does not contain antibodies for the blood cell antigens of the donor. The "ABO system" is the most important blood-group system in human-blood transfusion. The associated anti-A and anti-B antibodies are usually "immunoglobulin M", abbreviated IgM, antibodies. ABO IgM antibodies are produced in the first years of life by sensitization to environmental substances such as food, bacteria, and viruses. The original terminology used by Karl Landsteiner in 1901 for the classification was A/B/C; in later publications "C" became "O". Type O is often called "0" ("zero", or
ABO blood group system 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 called 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 ABO system. He asserted: Thus, he discovered two antigens (agglutinogens A and B) and two antibodies (agglutinins - anti-A and anti-B). His third group (C) indicated absence of both