High titer anti-U in pregnancy with absent hemolysis in a U-positive neonate: a case report and literature review

Objectives The high-prevalence U antigen is part of the MNS blood group system and is carried on glycophorin B along with the S and s antigens. Approximately 1.5% of the Black population has U-negative or U-variant phenotypes and can potentially develop IgG antibodies against the U antigen. Anti-U antibodies are clinically significant and associated with both transfusion reactions and hemolytic disease of the fetus and newborn (HDFN). Methods A 24-year-old G4P3003 woman with a history of anti-U alloimmunization had an anti-U titer of 1:16 identified during a second-trimester antibody screen at an outside hospital. Upon repeat testing during the third trimester, the titer had increased to 1:1024. The patient was transferred to our institution for further management, where a middle cerebral artery (MCA) Doppler ultrasound was performed, revealing a normal MCA peak systolic velocity for gestational age. Results A healthy female neonate, delivered at 37 weeks and 0 days, had a strongly positive direct antiglobulin test with anti-U identified in the eluate. The neonate did not require phototherapy or transfusions and was discharged on day 2 of life in stable condition. A human erythrocyte antigen genotyping panel confirmed a U+ phenotype. Conclusions This case demonstrates a passively acquired anti-U in a neonate, born to a mother with a high anti-U titer, who did not show any evidence of HDFN. To date, this is the highest anti-U maternal titer reported in the literature in which the neonate did not require any intervention.