Platelet immunology – an update

Abstract

The human platelet allo (HPA) antigen system has been well described with more than 16 HPA epitopes identified on platelet glycoproteins. There have been 6 bi-allelic systems characterised, HPA 1–5 and 15, which show variation in genotype frequency across populations. The very low frequency epitopes (HPA 6–14, 16, 17) may be a unique single nucleotide polymorphism (SNP) and have been identified through the characterisation of the specific antibody to the high frequency antigen. Although thought to be platelet specific, some of the HPA antigens have been identified on other cells and tissues. For example, GPIIIa has been detected on endothelial cells and fibroblasts. Allo-antibodies to the HPA system have been shown to be clinically significant in neonatal alloimmune thrombocytope-nia (NAIT) causing severe thrombocytopenia in the neonate due to a maternal antibody against the fetal paternally derived HPA antigen. Antibodies to the HPA 1a antigen are most commonly implicated in NAIT (75%), followed by antibodies to HPA 5,¹ but all HPA systems have been reported. NAIT has been the main source of HPA allo-antibodies identifying the majority of the HPA epitopes. This presentation will review the HPA allo-antigen systems and methods used to identify the platelet specific allo-antibodies.