ABO Rh Typing

The ABO system is the most clinically significant blood group system in transfusion medicine.

• Carbohydrate antigens
• Non-RBC stimulated: individuals possess ABO antibodies to the antigens that they lack
• IgM
• Cause severe transfusion reactions
  • Cell lysis and hemolysis

ABO Genes

• A and B genes present on chromosome 9
  • Code for transferase enzymes, which transfer sugar to a precursor on the RBC membrane
• A and B co-dominant, O is recessive
• One allele received from each parent
• Cis-AB inheritance
  • Rare situation where A and B genes end up on same chromosome during crossover
  • Can result in AB mother and OO father having AB baby

H Gene

• Present on chromosome 19
• H gene codes for a fucosyl transferase (FUT 1) that produces H antigen
  • Precursor to A and B antigens
• H allele frequency >99.99%
  • hh is Bombay phenotype
    • Produces anti-H antibodies

ABO Antigens

• Carbohydrates attached to RBCs via gene-encoded transferase
• A and B antigens are sugars attached to terminal end of oligosaccharides
• O group has H terminal antigen instead of A or B

	H	A	B
Gene Product (Enzyme)	L-fucosyltransferase	N-acetylgalactosaminyltransferase	D-galactosyltransferase
Dominant/Terminal Sugar	L-fucose	N-acetyl-galactosamine	D-galactose
Groups	A, B, AB, O	A, AB	B, AB

ABO Typing

• Babies: perform forward testing ONLY
• Babies: if baby appears Rh Negative and the mother is Rh Negative, perform weak D testing
• AB Rh Positive patients require Rh Control to be performed
  • RHC should be a reaction of 0. This checks that the agglutination seen is not a result of autoagglutination in the patient.
• Rh Control is also used for discrepancies (Rh testing results don't match, e.g. Anti-D1 is 2+ and Anti-D2 is 0)
• Rh Control is also used for IAT Testing (e.g., Weak D testing)