The ABO and Rh blood group systems are the most important blood group systems in transfusion medicine. ABO compatibility is the fundamental requirement for safe red blood cell transfusion, and Rh (D) typing is critical for preventing hemolytic disease of the fetus and newborn (HDFN).
ABO Antigens and Antibodies
ABO antigens are carbohydrate structures present on the surface of red blood cells and other tissues. The H antigen is the precursor: the A allele encodes a glycosyltransferase that adds N-acetylgalactosamine to H antigen to form A antigen; the B allele encodes a transferase that adds D-galactose to form B antigen; the O allele is a non-functional variant. ABO antibodies (isoagglutinins) are naturally occurring IgM antibodies produced without prior RBC exposure, directed against the missing ABO antigens. Group A individuals have anti-B, group B have anti-A, group O have both anti-A and anti-B, and group AB have neither. Anti-A and anti-B are capable of activating complement and causing rapid intravascular hemolysis, making ABO compatibility mandatory for transfusion.
ABO Typing Methods
ABO typing is performed by both forward typing (cell grouping) and reverse typing (serum grouping). Forward typing uses monoclonal anti-A and anti-B reagents to detect A and B antigens on patient RBCs by agglutination. Reverse typing uses A1 and B reagent RBCs to detect ABO antibodies in patient serum or plasma. The two methods must agree for the blood type to be confirmed. Discrepancies may arise from weak ABO subgroups (A2, A3, Ael, B3), cold agglutinins, rouleaux, recent ABO-incompatible transfusion, or hematopoietic stem cell transplantation. Resolution of discrepancies involves additional testing with anti-A1 lectin (Dolichos biflorus), saliva testing for secretor status, and adsorption-elution studies.
ABO Subgroups
A is the most common subgroup and reacts strongly with anti-A. A2 is the most common weak subgroup, comprising approximately 20% of A individuals; A2 RBCs may not agglutinate with anti-A1 lectin, and some A2 individuals produce anti-A1. Other rare A subgroups (A3, Ax, Ael) show progressively weaker expression. B subgroups are less common but exist. Subgroup identification is important for resolving ABO typing discrepancies and for component selection in transplantation.
Rh Blood Group System
The Rh system is the most complex and polymorphic blood group system, with over 50 antigens. The most important is D antigen (RhD), which is highly immunogenic. Rh-positive individuals express D antigen; Rh-negative individuals lack it. The Rh system also includes C, c, E, and e antigens, which are encoded by two homologous genes: RHD (encoding D) and RHCE (encoding C/c and E/e). Unlike ABO, Rh antibodies are immune antibodies (IgG) formed after exposure through transfusion or pregnancy. Anti-D is the most common cause of HDFN.
Rh Typing
RhD typing is performed using monoclonal anti-D reagents. For donor units, both a strong and weak D test are performed to detect weak D (partial D) expression. Weak D phenotypes arise from reduced D antigen density or qualitative variants, and individuals may produce anti-D if exposed to normal D antigen. For patient testing, anti-D reagents that detect weak D are used. Discrepancies between typing methods, especially in recently transfused patients or after hematopoietic stem cell transplantation, require molecular genotyping for resolution.
Compatibility Testing
Before transfusion, ABO and Rh typing of both donor unit and recipient is required. Group O RBCs are the universal donor (no A or B antigens), while group AB plasma is the universal plasma donor (no anti-A/anti-B). Rh-negative patients should receive Rh-negative RBCs to avoid anti-D sensitization, especially women of childbearing potential. Antibody screening and crossmatching are performed to detect clinically significant antibodies beyond ABO and Rh.
Hemolytic Disease of the Fetus and Newborn
HDFN occurs when maternal IgG antibodies cross the placenta and destroy fetal RBCs. The most common cause is RhD incompatibility (RhD-negative mother carrying an RhD-positive fetus). Prevention with anti-D immunoglobulin (RhoGAM) at 28 weeks gestation and within 72 hours of delivery effectively reduces sensitization. ABO-HDFN is more common but generally milder, occurring in group O mothers with group A or B infants. Laboratory investigation includes maternal antibody screening and titer, fetal/maternal genotyping, and assessment of neonatal anemia and hyperbilirubinemia.
