The prominent expression of the immunoinhibitory “no danger-signal”, CD200 in the HF bulge [59] not only demarcates ePCs [16,21,53], but also constitutes part of the relative immune privilege of the HF bulge, which may protect the HF eSC niche against autoimmune attacks and is characterized by an extremely low expression of major histocompatibility complex (MHC) class Ia [21,60,61]. Here, CD200 is linked to hydrops fetalis.