the u antigen

What is the U antigen?

The U antigen is a ‘high incidence’ antigen, present on the red cells of 99% of Black and virtually 100% of Caucasian people.

Glycophorin B forms the stem for antigens S and s.

U-negative individuals have absent or altered gylcophorin B.

If you have U, you may or may not have S or s.

If you don’t have U, you can’t have S or s, and could form Anti-U alloantibodies.

Anti-U can target the U antigen regardless of whether S or s is present.

example scenario


Qu. A woman with sickle cell anaemia is found to have an Anti-U alloantibody during her pregnancy booking bloods. She does not currently require blood. Later in the pregnancy she is found to have placenta praevia. What does this mean for the patient and what will you do?




  • Monitor titre (serial dilution IAT)

  • USS for MCA dopplers, amniotic fluid volume

  • Refer fetomaternal unit, MDT approach


  • Planned C-section

  • Pre-op anaemia Mx

  • Cell salvage

Sourcing blood:

  • 1st choice – Is there any fresh U neg in stock?

  • 2nd choice - Blood from a directed donor – fresh, longer shelf-life, better quality.

  • 3rd choice – Frozen blood bank


Would you use relatives as directed donors?

Risk is the increased pressure to lie on the donor safety check forms.

Would need irradiating up to 2nd degree relatives. 



At risk of HDFN. (Anti-U can target U antigen even buried beneath S or s.)

  • Cord blood at birth – DAT (—> elution if positive), Group, FBC, Bilirubin

  • Monitor for jaundice

  • Will need U neg blood at birth if transfused due to maternal antibody