The ABO blood type system classifies human blood into four main groups: A, B, AB, and O. These groups are determined by the presence or absence of specific antigens on the surface of red blood cells. Type A has A antigens, type B has B antigens, type AB has both, and type O has neither. Blood type is important for blood transfusions, organ transplants, and pregnancy. Mismatched blood types can lead to serious immune reactions. Additionally, knowing your blood type can be useful for medical emergencies and certain health conditions.