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
Rh blood group system factor is not identical to the rhesus monkey factor; but by then, "Rhesus Group" and like terms were already in widespread, worldwide use. Thus, notwithstanding it is a misnomer, the term survives (e.g. "rhesus blood group system", and the obsolete terms "rhesus factor", " rhesus positive", and "rhesus negative" - all three which actually refer specifically and "only" to the Rh D factor; and, thus, are misleading when unmodified). Contemporary practice is to use "Rh" as a term of art instead of "Rhesus"; viz. "Rh Group," "Rh factors," "Rh D," etc. The first rhesus blood type was discovered in 1937