CD4 and hydrops fetalis: While MHC-I expression may be low in the healthy hair bulb epithelium it is not fully absent, and there are also low numbers of various immunocytes (e.g., CD4+ and CD8+ T cells, macrophages, Langerhans, mast cells) both in the HF mesenchyme (dermal sheath, dermal papilla) and even within the hair bulb itself (75).