The ABO blood group is a major blood classification system used all over the world. It is the foundation of crossmatching blood samples between donors and recipients to find blood that is safe to transfuse. Before the development of understanding about blood groups, blood transfusions were extremely dangerous and often failed because the wrong type of blood was transfused.
Development of the ABO blood group is credited to physician Karl Landsteiner. He noted that blood from some people appeared to cause clumping when mixed with blood from other people and theorized that rather than being universal, human blood was actually different between individuals. He released a study in 1909 outlining the ABO blood typing system and later won a Nobel Prize for his groundbreaking work, as it was a very significant contribution to medicine.
Membership in the ABO blood group is determined through genetic inheritance. Some people inherit a pair of genes known as A genes that code for the presence of A antigens on the surface of their blood cells. These individuals are said to have type A blood. Other people inherit B genes that code for the B antigen, and are in the B blood group. It is also possible to inherit one copy of each antigen from one's parents, resulting in AB blood.
Some individuals inherit neither A nor B genes. These individuals are said to have type O blood. Blood infused between different members of the ABO blood group can react, causing a transfusion reaction and severe illness or death. When blood is tested first for the presence of antigens, it can be held back from recipients who would react with it, and transfused into people who are a match for that blood type.
This blood grouping system is complicated by the presence or lack thereof of rhesus factor, another important component in blood. The presence of rhesus factor is indicated by stating whether someone has “negative” or “positive” blood and it influences the safety of blood transfusions. Rather than dividing people into A, B, AB, or O, doctors determine whether they are A-, A+, B-, B+, AB-, AB+, O-, or O+.
There are other blood group systems in addition to the ABO blood group, but this group is the most widely used and it catches most cases where a harmful transfusion reaction would occur. Other blood groupings are based on different antigens and antibodies found in the blood of some individuals and are considered in addition to, not instead of, the ABO grouping of the patient.