Anti-Bak(a) neonatal alloimmune thrombocytopenia: possible prevention by intravenous immunoglobulin.

Neonatal alloimmune thrombocytopenia (NAIT) most commonly involves antibodies directed against the PlA1 antigen, but other platelet specific alloantigens have been associated with it. We describe the case of a mother whose first three infants developed NAIT secondary to anti-Bak(a) antibodies, while her fourth infant did not. The three affected infants were treated postnatally with platelet transfusions. The fourth infant was treated antenatally with one dose of intravenous immunoglobulin (IVIg) given to the mother. Postpartum analysis revealed the infant's platelets to be Bak(a)-positive but negative for elevated IgG. Maternal serum reacted with neonatal platelets in vitro, but cord serum was negative for antiplatelet antibodies. These clinical observations do not prove the efficacy of IVIg; however, they raise several questions: Why wasn't this infant thrombocytopenic? Why didn't the umbilical cord contain maternal antibody? Was the single dose of IVIg responsible for preventing NAIT? IVIg is currently under investigation in a clinical trial evaluating its effectiveness in preventing NAIT in mothers with anti-PlA1 antibodies, where it has shown some success. There have been no reports of the use against anti-Bak(a) antibodies. We suggest that a weekly dose schedule may not be necessary for all affected pregnancies, and antibodies with specificity other than anti-PlA1 may require less vigorous therapy.