Fetal and neonatal alloimmune thrombocytopenia (AIT) is the most common cause of severe thrombocytopenia in fetuses and neonates. Maternal IgG alloantibodies against paternally derived fetal platelet antigens cross the placenta early in pregnancy and commonly result in severe thrombocytopenia. While the reported incidence varies somewhat with the assigned threshold of thrombocytopenia (50, 100, or 150 × 109/l), in most unselected populations, AIT affects 1 in 1000 to 1 in 2000 live births. Table 38.1 contains recent studies of AIT in unselected populations, systematically screened. In its severe form, AIT has the potential for significant morbidity (including intracranial hemorrhage) and mortality. In milder forms, there are either antibodies with no thrombocytopenia, or mild to moderate thrombocytopenia, which is identified only by a complete blood count obtained for another indication or in a screening study. While there have been extensive efforts made in the diagnosis and characterization of the disease, strategies for early detection and intervention remain controversial.

Pathogenesis

There are three requisite components of the pathogenesis of AIT. First, there must be an incompatibility between maternal and fetal ‘platelet-specific’ antigens, which are inherited from the father. The second requirement, alloimmunization, is a maternal humoral immune response specific to these ‘foreign’ fetal platelet antigens. Finally, maternal anti-platelet IgG alloantibodies must cross the placenta, bind to fetal platelet antigens, and cause fetal platelet destruction with resultant thrombocytopenia.