STEM, a new low-frequency Rh antigen associated with the e-variant phenotypes hrS-(Rh: -18, -19) and hrB-(Rh: -31,...

The new low-frequency antigen STEM was identified when a Cape coloured woman with an unknown antibody in her plasma (which agglutinated the red cells of her husband and a minority of other individuals) gave birth to a baby suffering mildly from haemolytic disease of the new-born. Most, but not all, examples of anti-STEM distinguish different strengths of STEM antigen on the red cells of different people; the different strengths are inherited. Family studies established that STEM was inherited as a Mendelian dominant character. STEM subdivides hrS-(Rh: -18, -19) and hrB- (Rh: -31, -34) red cells into two types: STEM+ and STEM-. The manually calculated lod score for STEM being associated with the Rh system is 3.91 and LIPED calculated lod score 4.35. The International Society of Blood Transfusion has allocated STEM the Rh number 49.