Thrombocytopenia is detected in around one percent of newborns. In otherwise healthy term newborns, thrombocytopenia is most often caused by alloantibodies transferred from the mother to the foetus. In the Caucasian populations human platelet antigen (HPA)-1a is the most immunogenic HPA. In Japan and China antibodies in the HPA-4 system are the most frequent cause of FNAIT. The immune response against the HPA must be understood in order to avoid immunization or prevent induction of FNAIT where mothers are already immunized.