SID: a new carbohydrate blood group system based on a well-characterized but still mysterious antigen of great pathophysiologic interest.

The high-prevalence blood group antigen, Sd^a, had been puzzling blood bankers and transfusionists for at least a decade when it was reported in 1967. The characteristic mix of agglutinates and free red blood cells (RBCs), caused by anti-Sd^a, is seen with the RBCs from 90 percent of individuals of European descent. However, only 2-4 percent of individuals are truly Sd(a-) and may produce anti-Sd^a. The antibodies, generally considered insignificant, may cause hemolytic transfusion reactions with high-expressing Sd(a+) RBCs (e.g., the unusual Cad phenotype, which can also be polyagglutinable). The Sd^a glycan, GalNAcβ1-4(NeuAcα2-3)Gal-R, is produced in the gastrointestinal and urinary systems, while its origin on RBCs is more controversial. According to current theory, Sd^a is likely to be passively adsorbed in low amounts, except in Cad individuals, where it has been found on erythroid proteins and at higher levels. The long-standing hypothesis that B4GALNT2 encodes the Sd^a synthase was confirmed in 2019, since homozygosity for a variant allele with rs7224888:C produces a non-functional enzyme associated with most cases of the Sd(a-) phenotype. Thereby, the SID blood group system was acknowledged as number 038 by the International Society of Blood Transfusion. Although the genetic background of Sd(a-) was settled, questions remain. The genetic background of the Cad phenotype has not yet been determined, and the source of the RBC-carried Sd^a is unknown. Furthermore, the interest of Sd^a stretches beyond transfusion medicine. Some tantalizing examples are lowered antigen levels in malignant tissue compared with normal tissue and interference with infectious agents like Escherichia coli, influenza virus, and malaria parasites.