Background

A recent systematic review showed that neonatal platelet counts do not appear to differ between suspected cases of FNAIT with and without HPA-5b antibodies (Alm J et al, Br J Haematol 2022). Given the high prevalence of anti-HPA-5b antibodies in unselected pregnant European women, the likelihood of incidental detection is high. Here, we report the natural course of HPA-5b-incompatible pregnancy in a 22-year-old woman with HPA-5b antibodies who was not treated with intravenous immunoglobulins.

Methods

All platelet antibody detections and crossmatch procedure (paternal platelets and maternal serum) were performed by monoclonal antibody immobilization of platelet antigens assay (MAIPA) as described by Kiefel et al. 1987, using monoclonal antibodies against glycoprotein (GP) IIb/IIIa, GPIa/IIa, GPIb/IX, CD109, CD36, PECAM-1 and HLA Class I-antigen. Acid elution treatment of newborn platelets was performed as described previously (Hotchkiss AJ et al, Blood 1986).

Results

The newborn was delivered at 40th week of gestation (3,050 g body weight, 49 cm body length) after an uneventful pregnancy. His platelet count at birth was 285 G/l. He had no signs or symptoms of bleeding. His was genotyped as HPA-5ab. Anti-HPA-5b antibodies were present in the maternal blood, and a crossmatch between maternal serum and paternal platelets showed positive reactions with GP Ia/IIa exclusively. A direct MAIPA performed with neonatal platelets showed elevated IgG bound to GP Ia/IIa but normal levels for IgG bound to GP IIb/IIIa and GP Ib/IX. An eluate performed from neonatal platelets revealed the presence of anti-HPA-5b antibodies on the newborn"s platelets.

Conclusion

In line with current evidence, we did not observe an effect of anti-HPA-5b antibodies in an untreated, second pregnancy of an anti-HPA-5b immunized mother. Although the antibodies were transferred through the placenta and could be eluated from the neonatal platelets, they apparently did not induce platelet destruction.

We believe that anti-HPA-5b does usually not induce FNAIT. Larger prospective studies are needed to determine its clinical relevance.

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