The human hair follicle immune system: cellular composition and immune privilege

The immunology of the hair follicle, its relationship with the "skin immune system' and its role in hair diseases remain biological intriguing and cli nically important. In this study, we analysed the immunoreactivity patterns of 15 immunodermatological markers to determine the cellular composition a nd immune privilege of the human hair follicle immune system in anagen VI ( growth phase). The most prominent cells located in or around the hair folli cle were Langerhans cells, CD4+ or CD8+ T cells, macrophages and mast cells , whereas B cells, natural killer cells and gamma delta T cells were found wry rarely. Langerhans cells (CD1a+, major histocompatibility complex, MHC class II+), and T cells (CD4+ or CD8+) were predominantly distributed in th e distal hair follicle epithelium, whereas macrophages (CD68+, MHC class II +) and mast cells (Giemsa+) were located in the perifollicular connective t issue sheath. Transmission electron microscopy confirmed low numbers of imm une cells in the proximal hair follicle epithelium, and wry few macrophages and Langerhans cells were seen in the dermal papilla, Melanophages were ob served in the connective tissue sheath and dermal papilla, MHC class I (HLA -A, -Bt -C) and beta(2)-microglobulin immunoreactivity was found on most sk in cells, but was substantially reduced on isthmus keratinocytes and virtua lly absent in the proximal hair follicle epithelium. Apart from the absence of Fas ligand immunoreactivity, the sharply reduced numbers of T cells and Langerhans cells, and the virtual absence of MHC class I expression all su ggest that the anagen proximal hair follicle constitutes an area of immune privilege within the hair follicle immune system, whose collapse may be cru cial for the pathogenesis of alopecia areata.