title: Human platelet antigen (HPA)-specific immunoglobulin M antibodies in neonatal alloimmune thrombocytopenia can inhibit the binding of HPA-specific immunoglobulin G antibodies.
abstract: BACKGROUND
  A term baby with unexplained thrombocytopenia and a platelet (PLT) count of 14 × 109/L (maternal PLT count was 200 × 109/L) was investigated for neonatal alloimmune thrombocytopenia.
  
  
  METHODS
  Serologic investigations were performed using the PLT immunofluorescence test (PIFT), monoclonal antibody immobilization of PLT antigens (MAIPA), and a bead-based assay (BBA) with maternal sera taken up to 56 days postdelivery. One serum sample was also separated into "immunoglobulin (Ig)M-rich" and "IgM-depleted" fractions and tested for PLT-specific antibodies. The family was genotyped for HPA.
  
  
  RESULTS
  HPA-3a-specific IgM antibodies were detected in the PIFT and confirmed in the BBA. PLT-specific IgG HPA-3a antibodies were not detected in the MAIPA assay and BBA in the initial sample but were detected in both techniques in subsequent serum samples. Testing of IgM-rich and IgM-depleted fractions in the MAIPA assay revealed that IgG antibody binding of the IgM-depleted fraction was inhibited by approximately 50% when it was reconstituted with the IgM-rich fraction suggesting that the IgM antibodies blocked the binding of the IgG antibodies. This effect was not observed when the IgM-depleted fraction or untreated serum was diluted with elution buffer. Incompatibility for HPA-3 was identified between the mother and the infant. The infant received one HPA-1a, -5b negative neonatal PLT transfusion, and one random PLT transfusion, with satisfactory outcomes. Both units were later found to be HPA-3b3b.
  
  
  CONCLUSIONS
  HPA-3a IgM antibodies can inhibit PLT-specific HPA-3a IgG antibodies in the MAIPA assay.
authors.affiliation: H&I Department, IBGRL, NHSBT Filton, Bristol, UK.
authors.forename: Matthew
authors.initials: M
authors.lastname: Hopkins
authors.affiliation: H&I Department, IBGRL, NHSBT Filton, Bristol, UK.
authors.forename: Geoff
authors.initials: G
authors.lastname: Lucas
authors.affiliation: H&I Department, IBGRL, NHSBT Filton, Bristol, UK.
authors.forename: Anthony
authors.initials: A
authors.lastname: Calvert
authors.affiliation: H&I Department, IBGRL, NHSBT Filton, Bristol, UK.
authors.forename: Nina
authors.initials: N
authors.lastname: Bendukidze
authors.affiliation: IBGRL, NHSBT Filton, Bristol, UK.
authors.forename: Frances
authors.initials: F
authors.lastname: Green
authors.affiliation: Department of Haematology, Leicester Royal Infirmary, Leicester, UK.
authors.forename: Krishna
authors.initials: K
authors.lastname: Kotecha
authors.affiliation: H&I Department, IBGRL, NHSBT Filton, Bristol, UK.
authors.forename: Anthony
authors.initials: A
authors.lastname: Poles
mesh_terms: D016824:Antigens, Human Platelet / Q000276:immunology*; D001666:Binding Sites, Antibody / Q000276:immunology; D006801:Humans; D007074:Immunoglobulin G / Q000276:immunology*; D007075:Immunoglobulin M / Q000276:immunology*; D007159:Immunologic Tests; D007231:Infant, Newborn; D007518:Isoantibodies / Q000276:immunology*; D054098:Thrombocytopenia, Neonatal Alloimmune / Q000276:immunology*
chemical_list: C044561:3a alloantigen, human; D016824:Antigens, Human Platelet; D007074:Immunoglobulin G; D007075:Immunoglobulin M; D007518:Isoantibodies