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 decide whether they could conscientiously accept fractions used from blood for purposes such as vaccination. This position has been expanded on since; the pre-formatted Durable Power of Attorney form provided by the Watch Tower Society includes an option for Jehovah's Witnesses to "accept all fractions derived from any primary component of blood." In 1964, Jehovah's Witnesses were prohibited from obtaining transfusions for pets, from using fertilizer containing blood, and were even advised (if their conscience troubled them) to write to dog food manufacturers to verify that their products were blood-free. Later that year, it was stated that doctors or nurses