Group C blood is different in that it agglutinates with both A and B. This was the discovery of blood groups for which Landsteiner was awarded the Nobel Prize in Physiology or Medicine in 1930. (C was later renamed to O after the German Ohne, meaning without, or zero, or null.) Another group (later named AB) was discovered a year later by Landsteiners students Adriano Sturli and Alfred von Decastello without designating the name (simply referring it to as "no particular type"). Thus, after Landsteiner, three blood types were initially recognised, namely A, B, and C.Czech serologist Jan Janský was the first to recognise and designate four blood types in 1907 that he published in a local journal, using the Roman numerical I, II, III, and IV (corresponding to modern O, A, B, and AB respectively). Unknown to Janský, an American physician William L. Moss introduced almost identical classification in 1910; but his I and IV corresponding Janskýs IV and I. Moss came across Janskýs paper as his was being printed, mentioned it in a footnote. Thus the existence of two systems immediately created confusion and potential danger in medical practice. Mosss system was adopted in Britain, France, and the US, while Janskýs was preferred in most other European countries and some parts of the US. It was reported that "The practically universal use of the Moss classification at that time was completely and purposely cast aside. Therefore in place of bringing order out of chaos, chaos was increased in the larger cities."
The blood group may be included on identification tags or on tattoos worn by military personnel, in case they should need an emergency blood transfusion. Frontline German Waffen-SS had blood 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. Hemolytic disease of the newborn (HDN)
A pregnant woman may carry a fetus with a blood type which is different from her own. Typically, this is an issue if a Rh- mother has a child with a Rh+ father, and the fetus ends up being Rh+ like the father. In those cases, the mother can make IgG blood group antibodies. This can happen if some of the fetus blood cells pass into the mothers blood circulation (e.g. a small fetomaternal hemorrhage at the time of childbirth or obstetric intervention), or sometimes after a therapeutic blood transfusion. This can cause Rh disease or other forms of hemolytic disease of the newborn (HDN) in the current pregnancy and/or subsequent pregnancies. Sometimes this is lethal for the fetus; in these cases it is called hydrops fetalis.