ABO maternal-child discordance: Evidence of variable allelic expression and considerations for investigation.

ABO genotyping revealed an A transferase encoded by ABO*AW.29, with apparent variable activity. Although A antigen expression is well known to be weak in newborns, it was robust on the red blood cells (RBCs) of the AB infant and undetectable with anti-A on the mother. Variable expression of weak subgroups may reflect competition or enhancement by a codominant allele, as well as glycan chain maturation on red cells. Previous examples in group AB mothers with Aweak infants suggested that the decreased expression is primarily due to glycan immaturity. To our knowledge, this is the first reported case of the ABO*AW.29 allele presenting with weak A expression in a group Aweak mother and robust A expression in a group AB infant, suggesting the in trans allele is an important factor in determining transferase activity and may override age-related effects.