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     The image is immunohistochemistry of paraffin-embedded Human cervical cancer tissue using P11601(ABO Antibody) at dilution 1/50. (Original magnification: ×200)    

  
  

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     The image is immunohistochemistry of paraffin-embedded Human lung cancer tissue using P11601(ABO Antibody) at dilution 1/50. (Original magnification: ×200)    

  
  

ABO antibodies, also called anti-A and anti-B isohemagglutinins, are naturally occurring antibodies targeting blood group antigens absent in an individual. Discovered alongside the ABO blood group system in the early 1900s by Karl Landsteiner, these IgM-class antibodies form the foundation of transfusion medicine.

Normally present from infancy, they arise without prior exposure to foreign red blood cells. Type A individuals have anti-B antibodies, Type B have anti-A, Type O possess both, while Type AB lack them. This inverse relationship stems from immune tolerance to self-antigens and cross-reactivity with environmental antigens like gut bacteria polysaccharides.

Their clinical significance lies in mediating acute hemolytic transfusion reactions when incompatible blood is administered. Anti-A/B IgM activates complement cascades, causing rapid intravascular hemolysis. Additionally, IgG-class ABO antibodies (formed through sensitization) can cross the placenta, potentially causing hemolytic disease in newborns, though less severely than Rh incompatibility.

ABO compatibility remains critical in blood transfusions and organ transplantation. Notably, universal donor O-blood lacks A/B antigens, while universal plasma donor AB lacks antibodies. Modern techniques like antibody titration and immunoglobulin class analysis help mitigate risks in sensitive cases such as organ transplants and maternal-fetal incompatibility.