The cis-AB blood group phenotype: fundamental lessons in glycobiology.

The cis-AB phenotype can raise questions about an apparently paradoxical inheritance of the ABO blood group, such as the birth of an O child from an AB mother. These subtype ABO alleles confer the ability to create both A and B antigens with a single enzyme. A variety of different cis-AB enzymes have been reported and many feature an interchange of amino acids between the normal A enzyme sequence and its B counterpart, rendering the mutant enzyme capable of creating both antigens. The resulting red blood cells do not usually express A or B antigen at the same level that would be expected on common group A(1) or B red blood cells, and the results of investigations into the kinetics of the cis-AB enzyme more clearly predict the extent of antigen expression. By correctly identifying the cis-AB phenotype, the blood bank can be of assistance to a clinician faced with a patient with what appears to be a genetically impossible ABO blood group.