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) 5a-alloimmunized HPA-5b5b mother. The children were obligatory heterozygotes for HPA-5a but despite IgG class maternal anti-HPA-5a antibodies only two (second and fifth) of the six children developed neonatal thrombocytopenia. Throughout the 4-year follow-up the mother had anti-HPA-5a antibodies (confirmed in the 8th Platelet serology workshop of International Society of Blood Transfusion in 1996). Antibodies against glycoproteins (GP) IIbIIIa or IbIX were not detected. Differences in the children's HPA type (HPA-1, -2, -3, -5) did not correlate with 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 polymorphisms (C807 and G873) associated with the low expression of GP Ia. Thus, the inheritance pattern of the silent polymorphisms (C807T and G873A) did not help in predicting the recurrence risk of thrombocytopenia in the offspring. More detailed comprehension of the natural history of NAIT would be necessary to enable directing fetal blood sampling to the cases at the highest risk of thrombocytopenia.