Maternal alloimmunization against fetal platelet antigens: a prospective study.

Neonatal alloimmune thrombocytopenia (NAIT) is induced by maternal alloantibodies to fetal platelet antigens. This prospective study was carried out to evaluate the incidence of anti-platelet antibodies in 933 mother-child pairs where the mother and child were typed for the human platelet antigens (HPA)-1, -2, -3, -5. Sera from mismatched mother-child pairs were screened for anti-platelet antibodies, anti-HLA class I and blood group ABO IgG antibodies. Platelet-specific antibodies were anti-HPA-3a in one and anti-HPA-5b in 17 neonates, respectively. All these neonates had normal platelet counts. One woman had autoreactive antibodies. Anti-HLA class I and anti-blood group A IgG antibodies were detected in five and four neonates, respectively, born with a platelet count < 150 x 10(9)/l. None of the 11 homozygous HPA-1b mothers became immunized against their heterozygous offspring. The maternal HLA-allotypes HLA-DR52 and -DR6, typically found in individuals immunized against HPA-1a and -5b, respectively, were found in three of 11 HPA-b/b nonresponders and eight of the anti-HPA-5b responders. The results indicate that a risk for NAIT due to HPA-2 and -3 alloimmunization is low. The HLA allotypes do not predict the risk for NAIT due to HPA-1 or -5 alloimmunization. Maternal anti-HPA-5b antibodies do not correlate with the platelet count in the neonate.