Neonatal alloimmune thrombocytopenia : current considerations

Some mothers produce antibodies to the platelet antigens of their fetuses. Exposure to these antigens may occur owing to prior transfusions or through feto-matemal hemorrhage during gestation or delivery. The sen­ sitizing antigen is usually an epitope of one of the glycoproteins (GP) found on the platelet membrane. Specific GPs act as receptors for factors impor­ tant in hemostasis, such as von Willebrand’s Factor (vWF), fibrinogen, fibronectin, and collagen. Molecular biological techniques have identified single base pair substitutions resulting in antigenic specificity owing to one amino acid difference in a particular GP. Human platelet antigen (HPA)la is the most antigenic of the GPs. Neonatal alloimmune thrombocytopenia (NAT) results when a mother lacking an antigen present on fetal platelets develops the specific antibody. The incidence of NAT ranges from 1/1,500 to 1/5,000 births. An affected child is born with thrombocytopenia and may suffer consequences varying from petechiae and minor bleeding to central nervous system hemorrhage and death. Approximately 50 percent of the time, NAT is evident with the first pregnancy. Recent advances in obstet­ rical care permit percutaneous umbilical blood sampling (PUBS) early in pregnancy to determine whether or not the fetus is being adversely affected. Treatment through the use of cordocentesis and infusion of the mother’s platelets or other compatible platelets may be performed. Clear identification of antibodies against platelets remains problematic for the routine clinical laboratory. Reagents to identify platelet antigens and anti­ bodies are not readily available. Postnatal treatment of NAT requires infu­ sion of the mother’s platelets or platelets which are antigenically compati­ ble with the mother. tinctfrom the carbohydrate antigens (e.g., ABH) and Class I HLA antigens shared with other cells. Some of the plateletspecific an tig en ep ito p es are also expressed on endothelial cells and fibro­ blasts. There are five main systems of human platelet antigens (HPA) which have been standardized by the Interna­ tional Society of Blood T ransfusion Introduction Multiple antigen systems have been described for platelets.1,2 These plateletspecific antigens are separate and dis* Address reprint requests to Armand B. Glassman, M.D., Division of Laboratory Medicine, Uni­ versity of Texas, M.D. Anderson Cancer Center, 1515 Holcombe Blvd., Houston, TX 77030.