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What is the H Antigen? 

By Jack Loren 

September 21, 2023

In ABO blood group system, antigen A and antigen B are polymucosaccharides. 

Their carbohydrate chain is bound to the lipid molecule on the surface of plasma membrane of RBCs.  

The terminal monosaccharide molecule of these carbohydrate chains is different in antigen A and B

In is N-acetyle-α-D-galactosamine in antigen A and α-D-galactosamine in antigen B

The allele LA directs synthesis of enzyme N-acetyl galactosaminyl transferase which controls the addition of modified N-acetyl-α-D galactosamine to the carbohydrate chain.  

Allele LB controls synthesis of enzyme glactosyl transferase, which determines the addition of α-D-galactosamine. 

Both antigen A and B are synthesized from a precursor, called H-substance

This substance lacks both the terminal monosaccharides found in antigen A and B

Another pair of alleles., HH or Hh converts the precursor into antigen H