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
Jehovah's Witnesses and blood transfusions and other blood-borne infections, or immune system reactions. Many physicians have expressed a willingness to respect patients' preferences and provide bloodless treatment and about 200 hospitals offer bloodless medicine and surgery programs for patients who wish to avoid or limit blood transfusions. Bloodless surgery has been successfully performed in significant procedures including open-heart surgery and total hip replacements. A 2012 study in "JAMA Internal Medicine" concluded that "Witnesses do not appear to be at increased risk for surgical complications or long-term mortality when comparisons are properly made by transfusion status. Thus, current extreme blood management strategies do not appear to