ABOryhmä

ABO-ryhmä, or the ABO blood group system, is a classification of human blood based on the presence or absence of A and B antigens on the surface of red blood cells and the corresponding antibodies in plasma. The system is central to transfusion medicine, organ transplantation, and some aspects of obstetrics and forensic science. Individuals typically have one of four main blood groups: A, B, AB, or O. In addition to the ABO antigens, individuals may carry the RhD antigen, giving Rh-positive or Rh-negative phenotypes.

The A and B antigens are produced by enzymes encoded by the ABO gene on chromosome 9. The O group results from an inactive enzyme, so only the unmodified H antigen is present. Some individuals have the Bombay phenotype, which lacks H antigen and produces antibodies against H, A, and B, creating special transfusion requirements. Inheritance follows a co-dominant pattern for IA and IB alleles, with the i allele recessive to both; O corresponds to ii.

Clinically, ABO compatibility is crucial for safe transfusions and organ transplants. The presence of preformed anti-A and anti-B antibodies in plasma can trigger hemolytic reactions if incompatible blood is transfused. ABO typing is usually performed with forward typing (cell-based) and reverse typing (antibody-based), followed by crossmatching. The universal donor for red cell transfusion is often O negative, while the universal recipient for red cells is AB positive; these concepts have caveats in practice, especially for plasma transfusion and rare phenotypes.

Beyond transfusion, ABO antigens are found on various tissues and secretions in some individuals (secretor status), and the system has applications in anthropology and forensic science.