Neonatal alloimmune thrombocytopenia due to anti-HPA-2b (anti-Koa).

: Most severe cases of neonatal alloimmune thrombocytopenia (NAIT) are due to anti-HPA-1a (anti-PlA1) antibodies. We report a case of NAIT due to anti-HPA-2b that resulted in in utero intracranial hemorrhage.A 33-year-old G2P1A0 Caucasian woman had a routine ultrasound at 34 weeks. The fetus appeared to have a left hemispheric hematoma. IVIG, 1g/kg, was started immediately and administered weekly until delivery. One day after receiving the first dose of IVIG, fetal platelet count was 18 x 10(9)/L, and Hb was 116 g/L. Eleven mL of matched platelets compatible by monoclonal antibody immobilization of platelet antigens (MAIPA) assay were transfused in utero, raising the platelet count to 62 x 10(9)/L. Repeat transfusions were done later that week and 1 week later, with pretransfusion counts of 19 x 10(9)/L and 16 x 10(9)/L, respectively. Delivery by C section was done at 35.5 weeks, after the third platelet transfusion. Platelet count at birth was 77 x 10(9)/L. Drainage of the hematoma was performed after transfusion. Testing with a solid phase ELISA revealed reactivity against GP1b/IX. MAIPA testing after platelet treatment with the protease inhibitor leupeptin demonstrated the presence of anti-HPA-2b. On PCR-SSP the mother was HPA-2a homozygous, the father was HPA-2a/2b. Antibodies against the HPA-2b antigen located on the GP1b/IX complex have been reported in rare cases of NAIT. Testing is complicated by proteolytic degradation of the antigen-bearing fragment. Compatible platelets are easily found since approximately 85 percent of donors are HPA-2a/2a.