Fetal (but not maternal) FcRn is required for FNIT. Platelet counts in pups from breeding cages of (A) immunized β3^−/−FcRn^−/+ × β3^+/+FcRn^−/− and (B) immunized β3^−/−FcRn^−/− × β3^+/+FcRn^−/+. n = 4-9 for each group. (C) Circulating anti-β3 integrin and platelet-associated IgG were detected in β3^−/+FcRn^−/+ pups by flow cytometric analysis. n = 8-14 for each group.