The ABO system is a blood group classification system. The ABO system recognizes four types of blood groups, A, B, AB, and O. Each blood group has its unique antigen found on the red blood cells and antibodies in the plasma. Synonyms for the ABO system include the ABO grouping system, ABO blood typing, and ABO blood group system. Researchers and medical professionals might also use the term ABO blood grouping or typing. The ABO system is critical in transfusions because people can develop serious complications if their body rejects the blood from the wrong ABO group. Thus, it is essential to know your blood type before undergoing any medical procedure that involves blood transfusion.