ABO blood grouping is a fundamental blood test that determines a person's blood type based on the presence or absence of specific antigens (A and B) on the surface of red blood cells. The ABO system classifies blood into four main types: Type A (has A antigens), Type B (has B antigens), Type AB (has both A and B antigens), and Type O (has neither A nor B antigens). This classification is crucial for safe blood transfusions, as incompatible blood types can trigger severe immune reactions. The test also helps identify potential ABO incompatibility between expectant mothers and their babies, which can lead to hemolytic disease of the newborn. ABO grouping is routinely performed before surgery, during pregnancy, for blood donors, and in emergency situations to ensure proper blood matching and prevent transfusion complications.