STAMBP and hydrops fetalis: These ‘stressed’ day 1 HFs temporarily up-regulate MHC class Ia and ß2-microglobulin while the expression of IP guardians, that is αMSH and TGFβ2 remain unchanged (Uchida et al., 2021), indicating a transiently weakened, but partially maintained HF immune privilege (Bertolini et al., 2020; Ito et al., 2004).