h blood group system, system for classifying blood groups according to the presence or absence of the Rh antigen, often called the Rh factor, on the cell membranes of the red blood cells (erythrocytes). The designation Rh is derived from the use of the blood of rhesus monkeys in the basic test for determining the presence of the Rh antigen in human blood. The Rh blood group system was discovered in 1940 by Karl Landsteiner and A.S. Weiner. Since that time a number of distinct Rh antigens have been identified, but the first and most common one, called RhD, causes the most severe immune reaction and is the primary determinant of the Rh trait.