<P> The ABO blood types were first discovered by an Austrian Physician Karl Landsteiner working at the Pathological - Anatomical Institute of the University of Vienna (now Medical University of Vienna). In 1900, he found that blood sera from different persons would clump together (agglutinate) when mixed in test tubes, and not only that some human blood also agglutinated with animal blood . He wrote a two - sentence footnote: </P> <P> The serum of healthy human beings not only agglutinates animal red cells, but also often those of human origin, from other individuals . It remains to be seen whether this appearance is related to inborn differences between individuals or it is the result of some damage of bacterial kind . </P> <P> This was the first evidence that blood variation exists in humans--it was believed that all humans have similar blood . The next year, in 1901, he made a definitive observation that blood serum of an individual would agglutinate with only those of certain individuals . Based on this he classified human bloods into three groups, namely group A, group B, and group C. He defined that group A blood agglutinates with group B, but never with its own type . Similarly, group B blood agglutinates with group A. 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 . In his paper, he called the specific blood group interactions as isoagglutination, and also introduced the concept of agglutinins (antibodies), which is the actual basis of antigen - antibody reaction in ABO system . He asserted: </P> <P> (It) may be said that there exist at least two different types of agglutinins, one in A, another one in B, and both together </P>

Explain the basis of a/b/o and rh incompatibilities