ALLOIMMUNIZATION TO THE PLA1 PLATELET ANTIGEN - RESULTS OF A PROSPECTIVE-STUDY

The natural history of alloimmunization to the PlA1 platelet antigen is uncertain. We followed 50 PlA1‐negative pregnant women during pregnancy and for 6 months post‐partum in order to determine this natural history. The cohort of PlA1‐negative women was obtained by PlA1 typing 5000 women. Three PlA1‐negative women formed anti‐PlA1 antibodies during this prospective study, two in pregnancy and one in the immediate post‐partum period. All three PlA1 antibody producers were HLA‐DR3 positive, a histocompatibility phenotype that is strongly associated with alloimmunization to the PlA1 antigen. One of the three infants delivered to these mothers was thrombocytopenic (platelet count 9 x 109/1). The remaining two infants had normal platelet counts at birth (160 and 174 x 109/1). The HLA‐A1, ‐B8, ‐DR3 and ‐DRw52 phenotype frequencies in the group of PlA1‐negative women who did not form PlA1 antibodies (n= 47) was similar to that found in their husbands, and that expected in a normal Caucasian population. From our data we estimate that alloimmunization to the PlA1 antigen occurs in approximately one out of every 1000 pregnancies in a Caucasian population. It is important to recognize that not all pregnancies in which a mother has formed PlA1 alloantibodies will result in the delivery of a thrombocytopenic infant. These findings are relevant to programs designed to either prevent alloimmunization to the PlA1 antigen (through passive administration of anti‐PlA1 immunoglobulin to at‐risk PlA1‐negative mothers), or to identify women at risk of delivery of thrombocytopenic infants (by antenatal screening to detect women alloimmunized to the PlA1 antigen). Copyright © 1990, Wiley Blackwell. All rights reserved