Unusual Inheritance in the ABO Blood Group System: A Group O Child from A Group A 2 B Mother

. A serologically typical group A 2 B mother with no unusual anti‐A or anti‐B agglutinins delivered a normal group O infant with no demonstrable A or B antigens. By the age of 1 year the child had developed anti‐A and anti‐B agglutinins. He had H antigen and secreted H substance in his saliva. A second group Ochild has since been born. Although family studies were inconclusive, the most likely explanation is that the mother is a cis ‐AB. If this is the case, she differs from other reported cis ‐ABs in her apparently normal B antigen and the lack of anti‐B antibodies in her serum.