<P> The ABO blood group system is used to denote the presence of one, both, or neither of the AB antigens found on epithilia tissue and cells other than erythrocytes . In human blood transfusions it is the most important of the 35 different blood type (or group) classification systems currently recognized . A very rare (in modern medicine) miss - match in this, or any other serotype, can cause a serious, potentially fatal, adverse reaction after a transfusion, or an contra - indicated immune response to a organ transplant . The associated anti-A and anti-B antibodies are usually IgM antibodies, which are produced in the first years of life by sensitization to environmental substances, such as food, bacteria, and viruses . ABO blood types are also present in some other animals, for example rodents and apes, such as chimpanzees, bonobos, and gorillas . </P> <P> The ABO blood group system is widely credited to have been discovered by the Austrian scientist Karl Landsteiner, who identified the O, A, and B blood types in 1900 . Landsteiner originally described the O blood type as type "C", and in parts of Europe it is rendered as "0" (zero), signifying the lack of A or B antigen . Landsteiner was awarded the Nobel Prize in Physiology or Medicine in 1930 for his work . Alfred von Decastello and Adriano Sturli discovered the fourth type, AB, in 1902 . </P> <P> It was subsequently found that the Czech serologist Jan Janský had independently pioneered the classification of human blood into four groups in 1907, but Landsteiner's independent discovery had been accepted by the scientific world while Janský remained then in relative obscurity . However, in 1921 an American medical commission acknowledged Janský's classification . Jan Janský is nowadays credited with the first classification of blood into the four types (I, II, III, IV). </P> <P> Janský's classification remains in use today . In Russia and states of the former USSR, blood types O, A, B, and AB are respectively designated I, II, III, and IV . The designation A and B with reference to blood groups was proposed by Ludwik Hirszfeld . </P>

What are the four blood types in the abo system