ABO blood group system

The ABO blood group system classifies blood by whether red blood cells display A antigen, B antigen, both, or neither. These patterns define the A, B, AB, and O phenotypes.

ABO gene: A gene on chromosome 9 that encodes a glycosyltransferase determining the A or B antigen. Its alleles encode enzymes that build the defining A and B antigens.
Blood transfusion: The transfer of blood or blood components into a recipient's circulation. ABO matching is a central safety requirement for red-cell transfusions.
Karl Landsteiner: An Austrian physician and immunologist who identified the main human ABO blood groups. His experiments established the classification that bears the system's name.
Rh blood group system: A blood-group system defined chiefly by the presence or absence of the RhD antigen. Unlike ABO, Rh antibodies usually arise after exposure to foreign red cells or fetal cells.
H antigen: A precursor carbohydrate structure on red blood cells that A and B enzymes modify. ABO enzymes convert this shared precursor into A or B antigen.
Crossmatching: Laboratory testing for incompatibility between donor red cells and recipient plasma before transfusion. It supplements ABO typing by checking for clinically significant incompatibility.
Landsteiner's law: A rule stating that people generally have antibodies against ABO antigens absent from their own red cells. It summarizes the reciprocal antigen–antibody pattern underpinning ABO typing.
Kell blood group system: A blood-group system whose antigens can provoke clinically significant transfusion and pregnancy reactions. Kell incompatibility matters, but its antibodies are generally exposure-induced rather than naturally present.
ABO antibodies: Antibodies, mainly IgM, directed against A or B antigens absent from a person's red cells. These antibodies explain why incompatible ABO red cells can be destroyed rapidly.
Hemolytic disease of the newborn: Fetal or newborn red-cell destruction caused by maternal antibodies crossing the placenta. Maternal anti-A or anti-B can sometimes cause this disease, usually less severely than anti-D.

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