The natural history of maternal immunization against foetal platelet alloantigens

Summary.  Foetomaternal alloimmune thrombocytopenia (FMAIT) occurs when maternal antibodies of an antigen‐negative mother cause destruction of sensitized foetal platelets. In Caucasian populations, 6–12% of human platelet antigen (HPA)‐1a‐negative women develop anti‐HPA‐1a, and the incidence of clinically affected cases is estimated to be 10–20% of immunized women. This study was performed in order to elucidate the rate of maternal immunization, incidence of FMAIT and the likely outcome of the condition in Asians. Excluding two or more pregnancies during the period, serum samples from 24 630 pregnant women, mainly Japanese, were screened for antibodies against platelet alloantigens by means of mixed passive haemagglutination (MPHA) (Anti‐HPA‐MPHA, Olympus, Tokyo). Antibodies were detected in 0·91% (223/24 630) of the women's samples and the immunization rate was correlated with the number of pregnancies. Antibody specificity included anti‐HPA‐4b (49), anti‐HPA‐5a (three), anti‐HPA‐5b (168), anti‐HPA‐4b + 5b (one) and anti‐Naka (CD36) (two). No alloimmunization was observed within the HPA‐1, HPA‐2, HPA‐3 or HPA‐6 systems. Among HPA‐4b‐ or HPA‐5b‐negative women, 24% or 14% estimated, respectively, had antibodies and 26% (10/38) or 10% (12/125) of neonates, respectively, born to these mothers developed thrombocytopenia. Two neonates born to mothers having anti‐HPA‐4b developed generalized purpura. No cases of intracranial bleeding or death due to FMAIT were recorded. Generalized purpura due to FMAIT occurs in one in 9359 (95% CI: 1 in 77 519–1 in 2591) pregnancies solely because of HPA‐4b incompatibility.