FcRn a key factor in FNIT. (A) Fetal FcRn is required for FNIT. Low platelet counts are observed only in pups expressing fetal FcRn delivered from immunized β3^−/−FcRn^−/− × β3^+/+FcRn^−/+. (B) Anti-FcRn monoclonal antibody injection during pregnancy. Low platelet counts are observed only in pups delivered from nontreated immunized female mice. N = platelet counts. (Professional illustration by Paulette Dennis.)