Unusual inheritance in the ABO blood group system: a group O child from a group A2B mother.

A serologically typical group A2B 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 O child 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.