Effects of F/NAIT sera on HPA-1a/1b MKs and correlation between MK growth rates and platelet counts at birth. (A) Exposure to F/NAIT sera for 7 days caused similar decreases in the number of MKs generated from HPA-1a/1b and HPA-1a/1a cultures, compared with control sera. *P < .05. (B) Platelet counts at birth in neonates vs growth rate of MKs cultured in maternal sera (relative to controls). Samples known to have antibodies against class I HLA antigens are indicated by filled squares.