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
Blood type group tattoos during World War II. Rare blood types can cause supply problems for blood banks and hospitals. For example, Duffy-negative blood occurs much more frequently in people of African origin, and the rarity of this blood type in the rest of the population can result in a shortage of Duffy-negative blood for these patients. Similarly for RhD negative people, there is a risk associated with travelling to parts of the world where supplies of RhD negative blood are rare, particularly East Asia, where blood services may endeavor to encourage Westerners to donate blood. Pregnant women may carry a fetus