The role of the low‐frequency antigens in neonatal alloimmune thrombocytopenia

Abstract

Neonatal alloimmune thrombocytopenia (NAIT) results from specific maternal immunization against fetal platelet antigens inherited from the father which the mother lacks. The incidence of NAIT has been shown by prospective studies to be one out 800–1000 live births, but nevertheless still underdiagnosed. In Caucasians, human platelet antigen (HPA)‐1a is the most frequent antigen implicated in NAIT, followed by HPA‐5b. During recent years NAIT has been reported involving rare or private antigens, most of them located on the GPIIb‐IIIa complex. The description of such antibodies has been possible with the introduction of a routine maternal–paternal cross‐matching with an antigen‐capture assay and a panel of mouse monoclonal antigens in investigations of suspected cases of NAIT. At present 24 specific platelet alloantigens have been defined and the molecular basis for 22 out of 24 has been determined. Recent studies have shown that low‐frequency platelet alloantigens are not restricted to single families. Anti HPA‐9bw (Maxa) was found in two published series accounting for 2% of the confirmed NAIT cases and the neonates were severely affected. Three cases of HPA‐13bw families are reported to date and two cases for HPA‐8abw. The role of HPA‐15a or 15b in NAIT needs to be considered. The retrospective studies have shown a frequency highly variable from 2/1000 to 1·6%. The results of the different studies point out that laboratory testing for the main frequent alloantigenic system is no longer sufficient fully to evaluate NAIT in suspected cases. These investigations should be carried out in an experienced laboratory to ensure optimal testing. The frequencies of HPA are variable among populations, therefore the ethnic diversity must be taken into account in the context of the population migrations we may observe in our hospitals.