ABO type

ABO type

Updated: 07/29/2025

© Jun Wang, MD, PhD

 

General concept

  • Major blood type
  • Typing performed during pregnancy, before blood donation or transfusion, and transplant 
  • Identified by forward and reverse typing 
  • Depends on extracellular glycoprotein chain on red cells
  • Antibodies against absent antigens occur naturally
  • Usually anti-A or anti-B IgM
  • Low level of IgG might present, and cause hemolytic disease of the fetus and newborn

Genetics

  • Co-dominant ABO genes
  • Inherit one gene from each parent
  • Encode glucosyltransferases
  • Glucosyltransferases add different sugar to base extracellular glycoprotein chain

Gene H

  • Genetic base of ABO system
  •  Encodes alpha-2-L-fucosyltransferase
  • Add L-fucose to base extracellular glycoprotein chain
  • Form H antigen (H substance)

Gene A

  • Encodes alpha-3-N-acetylgalactosaminyltransferase
  • Add N-acetyl-D-galactosamine to H substance
  • Form A antigen

Gene B

  • Encodes alpha-3-D-galactosyltransferase
  • Add D-galactose to H substance
  • Form B antigen

Type O

  • No A nor B gene
  • No A nor B antigen
  • Only H substance
  • Anti-A and anti-B in plasma

Type A

  • Inherited A gene
  • A antigen on RBC
  • Anti-B in plasma
  • Transfuse with A or O RBCs

Type B

  • Inherited B gene
  • B antigen on RBC
  • Anti-A in plasma
  • Transfuse with B or O RBCs

Type AB

  • Inherited both A and B
  • Express both A and B antigens
  • Neither anti-A nor anti-B in plasma
  • Transfuse with A, B, or O RBCs

Type Bombay

  • Very rare 
  • Lack of H antigen 
  • Phenotyping as type O
  • Plasma contains strong anti-H
  • Agglutinate A, B, AB and O cells
  • When to suspect
    • Type O per ABO typing
    • Discrepancy in reverse typing with O cells (agglutination+)
    • Discrepancy in anti-body screening tests
  • Confirmed with anti-H lectin test (No agglutination with anti-H lectin)
  • Only accept Bombay type blood for transfusion

Interpretation of forward and reverse typing results

 

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