The ABO blood group system is a classification system for the antigens of human blood
discovered by Karl Landsteiner in 1900. There are four blood groups : A, B, AB and O.
The Rh blood group system is the second most significant system for blood grouping.
Rh factor refer to Rh D antigen only. Determination of Rh factor along with
ABO is essential for defining the Rh +ve or Rh -ve status of the individual.
Around 85% of the human population is Rh +ve while 15% is Rh -ve.
The ABO & Rh systems are the most significant blood group
systems from the clinical point of view.