Neonatal thrombocytopenia in two of six human platelet alloantigen (HPA) 5a-positive children of an HPA-5a-immunized mother

We describe a human platelet alloantigen (HPA) Sa-alloimmunized HPA-5b5b mo ther. The children were obligatory heterozygotes for HPA-Sa but despite IgG class maternal anti-HPA-Sa antibodies only two (second and fifth) of the s ix children developed neonatal thrombocytopenia. Throughout the 4-year foll ow-up the mother had anti-HPA-Sa antibodies (confirmed in the 8th Platelet serology workshop of International Society of Blood Transfusion in 1996). A ntibodies against glycoproteins (GP) IIbIIIa or IbIX were not detected. Dif ferences in the children's HPA type (HPA-1, -2, -3, -5) did not correlate w ith thrombocytopenia. We hypothesized that different expression of GPIaIIa recently associated with two silent polymorphisms (C807T and G873A) of GPIa could explain the unpredictable recurrence pattern of neonatal alloimmune thrombocytopenia (NAIT). Both parents were homozygous for the silent polymo rphisms (C807 and G873) associated with the low expression of GP Ia. Thus, the inheritance pattern of the silent polymorphisms (C807T and G873A) did n ot help in predicting the recurrence risk of thrombocytopenia in the offspr ing. More detailed comprehension of the natural history of NAIT would be ne cessary to enable directing fetal blood sampling to the cases at the highes t risk of thrombocytopenia.