In 1901, biologist Karl Landsteiner classified human blood based on the presence or absence of antigens in the blood cells and called ABO blood groups. ABO blood group is divided into four groups.
1. A group: 23% of blood is A group.
2. B group: 35% of blood is B group.
3. AB group: 8% of blood is AB group.
4. O group: 34% of blood is O group.