The ABO blood group system was discovered by Karl Landsteiner in 1901. By mixing the separated sera with suspensions of red cells obtained from the blood of different individuals, four patterns of agglutination were obtained. These patterns subdivide the population into four main blood groups (with approximate European Caucasian frequencies in parentheses): O (46.5 %); A (42 %); B (8.5 %); and AB (3 %).