THE Rh BLOOD GROUP SYSTEM

T he Rh blood group system is one of the most polymorphic and antigenic blood group systems. It is second only to ABO in importance in blood transfusion and is well known as a primary cause of hemolytic disease of the fetus and newborn (HDFN). The principal antigen is D, and the terms Rh positive and Rh negative refer to the presence or absence of D antigen. Caucasians of European extraction have the highest incidence of the Rh-negative phenotype (15% to 17%), and Rh-negative type is much less common in Africa (5%) and Australia and is considered a rare blood type not routinely tested for in some parts of Asia ( 1%). Other common Rh antigens include the antithetical C and c, and E and e antigens. Patients are not routinely typed for these unless they have developed atypical antibodies or are facing long-term transfusion support for diseases such as myelodysplasia or sickle cell disease (SCD). In addition to the five principal antigens (D, C, c, E, and e), more than 50 other Rh system antigens are known. Because they are not often encountered in the routine blood bank, they will not be covered here in detail. References at the end of the chapter provide further information for interested readers.