The ABO blood group system classifies blood based on the presence or absence of A and B antigens on the surface of red blood cells. Each blood type contains specific antibodies in the plasma that react against foreign antigens, which can lead to agglutination if incompatible blood types are mixed. Blood transfusions require careful matching between donor and recipient to prevent dangerous transfusion reactions. In addition to ABO groups, blood is also classified by the Rh factor, which depends on whether the D antigen is present on red blood cells. Blood typing is performed using laboratory tests such as forward grouping, which detects antigens on red blood cells, and reverse grouping, which identifies antibodies in plasma. Understanding blood types is essential for safe transfusions, pregnancy care, and many medical procedures