ABOSystem

The ABO system classifies human blood based on the presence or absence of A and B antigens on red blood cells and the corresponding antibodies in plasma. It was first described by Karl Landsteiner in 1901, enabling routine blood typing and safer transfusions.

The ABO gene, located on chromosome 9, encodes glycosyltransferases that modify the H antigen on red blood cells. The A allele adds N-acetylgalactosamine to H, the B allele adds galactose, and the O (i) allele yields an inactive enzyme, leaving H unmodified. The resulting phenotypes are A, B, AB, and O. In most individuals, naturally occurring anti-A and anti-B antibodies are primarily IgM and can cause agglutination and hemolysis if an incompatible transfusion occurs.

The H antigen is a common precursor to A and B antigens. A minority of individuals have the Bombay phenotype (hh), characterized by absence of H antigen on red cells and production of anti-H antibodies; these individuals require blood from hh donors. Secretor status, determined by the FUT2 gene, influences the presence of ABH antigens in secretions for many people, though red cell typing relies on H antigen status.

Clinically, ABO compatibility is a cornerstone of transfusion medicine; transfusing ABO-incompatible blood can trigger acute hemolytic reactions. In transplantation and pregnancy, anti-A and anti-B antibodies are considered for risk assessment. Distribution of ABO types varies by population. The system remains widely used in forensic serology and paternity testing.