Blood Group Genotyping

Blood group antigens are polymorphisms of proteins and carbohydrates on the outside surface of the red blood cell (RBC) (Fig. 1) and are defined by serum alloantibodies produced in response to an immunizing event such as transfusion or pregnancy. It is the antibody that causes clinical problems in transfusion incompatibility, maternal-fetal incompatibility, and autoimmune hemolytic anemia.

The major risks of transfusions are unexpected incompatibility reactions1-1-1 and the transmission of infectious agents. Iron overload and alloimmunization are also frequently observed among some categories of chronically transfused patients. Alloimmunization leads to an increased risk of transfusion reactions, reducing the available pool of compatible blood for transfusion in subsequent crises. Alloimmunization is the source of a variety of problems during long-term medical and transfusion management, with the main problem being the identification of appropriate antigen-negative RBCs for transfusion.[2]

Thus in transfusion medicine much time and effort are expended in detecting and identifying blood group antibodies. Next to ABO, the most clinically significant antibodies are those in Rh, Kell, Duffy, and Kidd blood

group systems.1

Red blood cell phenotyping is essential to confirm the identity of suspected alloantibodies and to facilitate the identification of antibodies that may be formed in the future. Accurate antigen typing of transfused patients is often a difficult task because of the presence of donor RBCs in the patients' circulation. Thus in these patients phenotyping can be time consuming and difficult to interpret. It is also complicated to type cells when a patient's RBCs have a positive direct antiglobulin test and no direct agglutinating antibody is available.

DNA technology led to the understanding of the molecular basis of many blood group antigens. The genes encoding 28 of the 29 blood group systems (only P remains to be resolved) have been cloned and sequenced1-4-71 (Table 1), which has permitted the elucidation of the molecular basis of many common blood group antigens. There are many molecular events that give rise to blood group antigens and phenotypes (Table 2); however, the majority of genetically defined blood group antigens are the consequence of a single-nucleotide polymorphism (SNP). (For current information regarding blood group antigen SNPs, the following website is recommended: htm). This knowledge allows the use of DNA-based assays to detect specific blood group SNPs and that can be used to overcome the limitations of hemagglutination

assays.1 ,

Several assays for blood group genotyping of patients have recently been developed to predict the blood group antigen profile of an individual, with the goal of reducing risk or helping in the assessment of the risk of hemolytic disease of the newborn (HDN);[10-16] they include PCR-RFLP, allele-specific PCR, sequence-specific PCR as single or multiplex assays, real-time quantitative PCR.

These assays can be applied for blood group antigens to type patients who have recently received transfusion; to type patients whose RBCs are coated with immunoglob-ulin; to identify a fetus at risk for HDN; to determine which phenotypically antigen-negative patients can receive antigen-positive RBCs; to type donors for antibody identification panels; to type patients who have an antigen that is expressed weakly on RBCs; to determine RHD"/>
Fig. 1 Model of red blood cell membrane components that carry blood group antigens. (View this art in color at

zygosity; to mass screen for antigen-negative donors; to resolve A, B, and D discrepancies.

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