Scianna antigen system

Scianna antigen system is a blood group system that includes several antigens, the most notable of which are Sc1 and Sc2. This system is of clinical importance in blood transfusion and pregnancy, as mismatches between donor and recipient in this system can lead to hemolytic transfusion reactions and hemolytic disease of the newborn (HDN).

Overview

The Scianna antigen system is defined by the presence of specific antigens on the surface of red blood cells (RBCs). These antigens are genetically determined and are encoded by the ERMAP gene located on chromosome 1. The system is named after the first identified antibody, anti-Sc1, in the serum of a patient named Scianna. The two primary antigens, Sc1 and Sc2, exhibit codominant inheritance. A third antigen, Sc3, is a high-prevalence antigen within this system.

Genetics

The Scianna antigens are encoded by the ERMAP gene, which stands for erythroblast membrane-associated protein. Variations in this gene lead to the different phenotypes observed in the population. The Sc1 and Sc2 antigens result from a single nucleotide polymorphism (SNP) within the ERMAP gene, leading to an amino acid substitution that alters the antigenic properties of the protein.

Clinical Significance

The Scianna antigen system is of particular interest in the context of blood transfusion and pregnancy. Although antibodies to Scianna antigens are relatively rare, when present, they can cause hemolytic transfusion reactions. In pregnancy, the presence of maternal antibodies against fetal Scianna antigens can lead to hemolytic disease of the newborn (HDN), although such cases are rare.

Blood Transfusion

In blood transfusion practices, it is essential to match donor and recipient blood types to prevent adverse reactions. While the ABO and Rh systems are the most critical to match, antibodies against antigens in other systems, including Scianna, can cause transfusion reactions. Therefore, screening for antibodies against Scianna antigens may be performed in patients with a history of transfusion reactions or in those with antibodies of unknown specificity.

Pregnancy

During pregnancy, the maternal immune system can produce antibodies against fetal red blood cell antigens inherited from the father and not present in the mother, including those in the Scianna system. These antibodies can cross the placenta and cause hemolytic disease of the newborn (HDN), leading to anemia and hyperbilirubinemia in the newborn. However, HDN due to Scianna antibodies is rare.

Phenotypes and Antigens

The Scianna blood group system includes several phenotypes, determined by the presence or absence of the Sc1 and Sc2 antigens. The most common phenotypes are Sc(1+2-), Sc(1-2+), and Sc(1+2+). The Sc(1-2-) phenotype is very rare and has been associated with the null phenotype, in which the ERMAP gene is inactive or deleted, leading to the absence of Scianna antigens on the red blood cells.

Conclusion

The Scianna antigen system plays a role in transfusion medicine and maternal-fetal medicine, although its antigens are less commonly implicated in clinical complications than those of the ABO or Rh systems. Understanding the genetics and clinical implications of the Scianna system can aid in the prevention and management of transfusion reactions and hemolytic disease of the newborn.

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