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 Dr. Karl Landsteiner in 1901 for the classification is A/B/C; in later publications "C" became "O". "O" is often called 0 (zero, or null) in other languages. The Austrian Federal Ministry of Health claims the original terminology used by Dr. Karl Landsteiner in 1901 for the classification is 0(Zero)/A/B/AB and that in later publications "0" became "O" in most of English language countries.
Blood group AB individuals have both A and B antigens on the surface of their RBCs, and their blood plasma does not contain any antibodies against either A or B antigen. Therefore, an individual with type AB blood can receive blood from any group (with AB being preferable), but cannot donate blood to any group other than AB. They are known as universal recipients.
Blood group A individuals have the A antigen on the surface of their RBCs, and blood serum containing IgM antibodies against the B antigen. Therefore, a group A individual can receive blood only from individuals of groups A or O (with A being preferable), and can donate blood to individuals with type A or AB.
Blood group B individuals have the B antigen on the surface of their RBCs, and blood serum containing IgM antibodies against the A antigen. Therefore, a group B individual can receive blood only from individuals of groups B or O (with B being preferable), and can donate blood to individuals with type B or AB.
Blood group O (or blood group zero in some countries) individuals do not have either A or B antigens on the surface of their RBCs, and their blood serum contains IgM anti-A and anti-B antibodies. Therefore, a group O individual can receive blood only from a group O individual, but can donate blood to individuals of any ABO blood group (i.e., A, B, O or AB). If a patient in a hospital situation needs a blood transfusion in an emergency, and if the time taken to process the recipient's blood would cause a detrimental delay, O negative blood can be issued. Because it is compatible with anyone, O negative blood is often overused and consequently is always in short supply. According to the American Association of Blood Banks and the British Chief Medical Officer’s National Blood Transfusion Committee, the use of group O RhD negative red cells should be restricted to persons with O negative blood, women who might be pregnant, and emergency cases in which blood-group testing is genuinely impracticable.
