Neonatal alloimmune thrombocytopenia (NAIT) refers to a disorder in which fetal platelets contain an antigen inherited from the father that the mother lacks. These antibodies cross the placenta and bind to the fetal platelets. Clearance of the antibody-coated platelets results in fetal/neonatal thrombocytopenia.
Antibodies directed against platelet antigen HPA-1a (formerly called PLA-1) account for more than 80 percent of NAIT in Caucasians . Approximately 98 percent of Caucasians express HPA-1a and about 2 percent of women are HPA-1a negative (HPA-1b homozygotes). Immunoglobulin G (IgG) antibodies (anti HPA-1a) are found in about 10 percent of HPA-1a negative pregnant women .
The HPA-5b antigen (formerly the Br antigen) is the second most common platelet antigen causing NAIT in Caucasians. In Asians, the HPA-4 system (formerly called Pen or Yuk) is the most frequent cause of NAIT. More than a dozen other platelet antigens have been associated with NAIT. The prevalence of these antigens varies among different ethnic groups; some antigens are quite rare .
The severity of NAIT differs depending on the antigen involved; HPA-1a causes severe disease . Other factors, such as HLA type, can modulate severity of disease in offspring. Vascular endothelial dysfunction has also been implicated in the pathogenesis of NAIT [4,5]. (See "Causes of neonatal thrombocytopenia", section on 'Fetal and neonatal alloimmune thrombocytopenia'.)
CLINICAL MANIFESTATIONS AND DIAGNOSIS
The mother of a fetus/newborn with NAIT is asymptomatic.