ABO and Rh Blood Group Systems

The ABO and Rh systems are the two most clinically important human red-cell blood-group systems. The ABO system is defined by the A and B carbohydrate antigens on the red-cell surface and by the reciprocal, naturally occurring anti-A and anti-B antibodies in plasma; the Rh system is defined by protein antigens, most importantly RhD, whose presence or absence makes a person RhD-positive or RhD-negative. Together they govern the compatibility of red-cell transfusions and the risk of immune hemolysis.

Definition

The ABO blood group system classifies red cells by the presence of A and/or B carbohydrate antigens with reciprocal plasma antibodies, and the Rh system classifies them by Rh proteins, chiefly the RhD antigen; both are the principal determinants of red-cell transfusion compatibility.

Scope

This entry covers the antigens and antibodies that define the ABO and Rh systems, the genetic and biochemical basis of the A, B, O, and RhD phenotypes, the rule of reciprocal naturally occurring ABO antibodies, and why these systems dominate transfusion compatibility and alloimmunization. It treats blood typing as a reference topic in immunohematology, not as transfusion instructions.

Core questions

• What antigens and antibodies define the ABO and Rh groups?
• Why does ABO incompatibility cause immediate, severe hemolysis?
• Why is RhD the most immunogenic of the Rh antigens?
• How do these systems determine which red cells may be transfused?

Key concepts

• A and B carbohydrate antigens
• Reciprocal naturally occurring anti-A and anti-B (Landsteiner's law)
• O phenotype and the H antigen
• RhD antigen and RhD-positive/negative status
• RhCE and the Rh haplotype
• Alloimmunization and the antiglobulin test
• ABO and Rh compatibility

Mechanisms

ABO antigens are carbohydrate structures built on a precursor (H) chain by glycosyltransferases encoded at the ABO locus; the A and B enzymes add different terminal sugars, while the O allele yields no active enzyme and leaves the H antigen unmodified. Individuals form antibodies against the A or B antigens they lack, so these antibodies are present without prior transfusion and can cause immediate intravascular hemolysis on ABO-incompatible transfusion. The Rh antigens are membrane proteins encoded by the RHD and RHCE genes; RhD is highly immunogenic, so RhD-negative individuals exposed to RhD-positive red cells may form anti-D, which is detected by the antiglobulin test and is central to hemolytic transfusion reactions and hemolytic disease of the fetus and newborn.

Clinical relevance

ABO and Rh typing is performed before red-cell transfusion and in pregnancy because mismatches in these systems are the main cause of acute hemolytic transfusion reactions and of RhD alloimmunization. This entry explains why these systems are tested and how incompatibility causes harm; it is educational and does not specify transfusion or prophylaxis decisions for individual patients.

Epidemiology

The distribution of ABO and RhD phenotypes varies markedly between populations, which has practical consequences for blood supply and the matching of donors to recipients. ABO is unique among blood groups in that essentially everyone carries antibodies to the antigens they lack, making ABO-incompatible red-cell transfusion uniformly dangerous.

History

Landsteiner described the ABO groups in 1900-1901, the discovery for which he later received the Nobel Prize, establishing the reciprocal antibody rule that bears his name. The Rh system was recognized in the 1940s through work by Landsteiner and Wiener and by Levine and Stetson on hemolytic disease, and the antiglobulin test introduced by Coombs, Mourant and Race in 1945 enabled reliable detection of Rh and other incomplete antibodies.

Key figures

• Karl Landsteiner
• Alexander Wiener
• Philip Levine
• Robin Coombs
• Marion Reid

Seminal works

• storry-2009
• avent-2000
• coombs-1945

Frequently asked questions

Why can people have anti-A or anti-B antibodies without ever receiving a transfusion?

Anti-A and anti-B are naturally occurring antibodies that develop early in life, probably in response to environmental antigens resembling A and B sugars, so a person makes antibodies against whichever ABO antigens they lack.

What does it mean to be RhD-negative?

It means the red cells lack the RhD protein antigen; RhD-negative individuals do not normally have anti-D but can form it after exposure to RhD-positive red cells, which matters for transfusion and pregnancy.