## ABO Blood Group System **Key Point:** The ABO antigen is a carbohydrate (oligosaccharide) structure attached to the red cell membrane via a glycoprotein or glycolipid backbone. It is the most clinically significant and immunogenic blood group system. ### Immunogenicity and Clinical Significance **High-Yield:** ABO antigens are naturally occurring and elicit IgM antibodies (anti-A and anti-B) without prior transfusion or pregnancy. This makes ABO incompatibility the most dangerous transfusion reaction. ### Comparison of Major Blood Group Systems | Feature | ABO | Rh | Kell | Duffy | |---------|-----|--|----|-------| | Antigen type | Carbohydrate | Protein | Protein | Protein | | Natural antibodies | Yes (IgM) | No | No | No | | Immunogenicity | Highest | High | Moderate | Low | | Hemolytic transfusion reaction | Immediate (IgM) | Delayed (IgG) | Delayed (IgG) | Delayed (IgG) | | Hemolytic disease of newborn | Yes (severe) | Yes (severe) | Rare | Rare | **Clinical Pearl:** ABO incompatibility causes immediate intravascular hemolysis due to IgM complement-fixing antibodies, whereas Rh incompatibility causes delayed extravascular hemolysis via IgG antibodies. **Mnemonic:** **CARD** = **C**arbohydrate (ABO), **A**ntibodies (natural), **R**eactions (immediate), **D**angerous — this is why ABO is the most critical to match before transfusion.
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