Abstrak HPA1a (n = 1),Methods: Sera from suspected FNAIT (n = 295) and PTR (n = 74) were collected in five years period (2008)(2009)(2010)(2011)(2012)(2013) and tested for the presence of platelet reactive antibodies by the use of antigen capture assay. Results:In 5/74 (5.41%) platelet specific antibodies against HPA-2b (n = 1), HPA-5a (n = 1), HPA-5b (n = 1), HPA-15b (n = 2) could be identified in our PTR cohort. In FNAIT cohort, platelet specific alloantibodies could be detected in 18 sera (6.10%) consisting anti-HPA-1a (n = 1), anti-HPA-3a (n = 3), anti-HPA-5a (n = 6), anti-HPA-5b (n = 6), anti-HPA-15a (n = 1), and anti-HPA-15b (n = 1). Conclusion:Our study indicates that anti-HPA-3, -HPA-5 and -HPA-15 antibodies seems to be the most platelet specific antibodies involved in FNAIT and PTR cases in Malaysian population. Since similar HPA allelic distribution among Malaysian and Indonesian populations have been observed, immunization against these three HPA systems are expected to be the most potential risk of alloimmune mediated platelet disorders in Indonesia.Keywords: alloimmune thrombocytopenia, transfusion pISSN: 0853-1773 • eISSN: 2252-8083 • http://dx.doi.org/10.13181/mji.v23i3.1011 • Med J Indones. 201423:158-62 Correspondence author: Sentot Santoso, sentot.santoso@immunologie.med.uni-giessen.deCl i n i c a l R e s e a r c h

Background and Objectives The foetal neonatal platelet alloimmune thrombocytopenia (FNAIT) is one of the most frequent clinical implications of immunization against human platelet antigen (HPA). Maternal IgG platelet alloantibodies induce the destruction of foetal platelets leading to thrombocytopenia. In Caucasian, alloantibodies against HPA‐1a are responsible for the majority of severe FNAIT. As the frequency of HPA‐1bb homozygous individuals is very low in Asia, the clinical relevance of anti‐HPA‐1a alloantibodies is doubtful in this population. Study design and methods Recently, we studied retrospectively sera from mothers with suspected FNAIT in Malaysia, a multi‐ethnic country. Surprisingly, we found in one maternal serum antibodies which may react with HPA‐1a. In this study, this FNAIT case, suspected to be caused by the anti‐HPA‐1a alloantibody, was investigated in more details. Results Antibody screening analysis using HPA‐phenotyped platelets by the antigen capture assay, MAIPA, showed specific reaction with the platelets GPIIb/IIIa carrying HPA‐1a antigenic determinants. In the control experiment, this antibody did not react with GPIIb/IIIa from HPA‐1bb individuals. In addition, no reaction was observed with other HPAs expressed on GPIIb/IIIa, GPIb/IX, GPIa/IIa, CD 109 and HLA Class I, both by screening tests and by cross‐match analysis (maternal serum against paternal platelets), in MAIPA. This result could be supported by the genotyping analysis of the family members (mother HPA‐1bb, father and the index child HPA‐1a positive). Conclusions Although considered rare, this case shows for the first time the clinical relevance of anti‐HPA‐1a antibody in the pathogenesis of FNAIT among Asian population.