Neonatal alloimmune thrombocytopenia

Neonatal alloimmune thrombocytopenia (or -paenia, NAITP or NAIT for short; or feto-maternal alloimmune thrombocytopenia or -paenia, FMAITP or FMAIT) is a disease that affects fetuses and newbornss. Genetic differences between the fetus and mother may result in the expression of certain antigens by fetal platelets, not expressed by the mother. Fetomaternal transfusions result in the recognition of these antigens by the mother's immune system as non-self, with the subsequence generation of allo-reactive antibodies which cross the placenta. NAIT, hence, is caused by transplacental passage of maternal platelet-specific alloantibody and rarely HLA allo-antibodies (which are expressed by platelets) to fetuses whose platelets express the corresponding antigens. It occurs in approximately one of 1,000 live births.

About 80% of cases of NAIT are caused by antibodies against platelet antigen HP-1a, 15% by anti-HP-5a, and 5% by other antibodies. Unlike the hemolytic disease, NAIT occurs during the first pregnancy in to 50% of cases, and the affected fetuses may develop severe thrombocytopenia (<50,000 /μL) very early during pregnancy. Usually, the thrombocytopenia increases as gestation progresses. In utero intracranial hemorrhage occurs in about 10% of affected cases. This complication may also take place before 20 weeks of gestation. The recurrence of NAIT been estimated to be more than 80% in subsequent pregnancies with incompatible fetuses.

Frequently, the thrombocytopenia is mild and the affected neonates remain largely asymptomatic. In these cases, therapeutic interventions are not indicated. In case of severe thrombocytopenia, the neonates may exhibit hemorrhagic complication at or a few hours after delivery. The most serious complication is intracranial hemorrhage, leading to death in approximately 10% or neurologic sequelae in 20% of cases.

The most rapidly effective treatment in infants with severe hemorrhage and/or severe thrombocytopenia is the transfusion of compatible platelets (i.e. platelets from a donor who, like the mother lacks the causative antigen).