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 life by sensitization to environmental substances, such as food, bacteria, and viruses. The ABO blood types were discovered by Karl Landsteiner in 1901, for which he received the Nobel Prize in Physiology or Medicine in 1930. ABO blood types are also present in some other animals such as rodents and apes, including chimpanzees, bonobos, and gorillas. The ABO blood types were first discovered by an Austrian Physician Karl Landsteiner working at the Pathological-Anatomical Institute of the University of Vienna (now Medical University of Vienna). In 1900, he found that blood sera from different persons would clump together (agglutinate) when mixed