CD4 and hydrops fetalis: A key concept in AA regards the the immune privilege of the HF (Bertolini et al. 2020; Paus et al. 2018): the HF epithelial cells express lower level of MHC I and MHC II molecules which reduce their immonogenecity; there is increased expression of immune-inhibitory molecules such as CD200 and PD-L1; the immune-suppressive local factors such as αMSH, TGFβ1, TGFβ2, IL-10, MIF are also protective; other local factors may include neuropeptides and neurohormones, CD4+ regulatory T cells, and peri-follicular mast cells.