Pathogenic model of alopecia areata. The normal hair follicle represents a site of immune privilege (IP). The guardians of IP include immunosuppressive cytokines such as α-melanocyte stimulating hormone (α-MSH), transforming growth factor-β (TGF-β), IK, indoleamine 2,3-dioxygenase (IDO), and IL-10. Patients with a specific genetic background are susceptible to developing alopecia areata (AA), most probably by downregulation of this immunosuppressive environment. It has been suggested that events such as stress, infection, or microtrauma might lead to downregulation of immunosuppressive cytokines. This downregulation enables the accumulation of natural killer (NK) cells around hair follicles. Furthermore, stress or other trauma may also alter the production of neuropeptides, including substance P (SP) and calcitonin gene–related peptide (CGRP). SP may upregulate the production of nerve growth factor, which in turn induces accumulation of mast cells around hair follicles. SP causes degranulation of mast cells, leading to a release of large amounts of TNF-α, which is known to inhibit hair growth. Furthermore, SP induces accumulation of CD8+ cells and induce these cells to produce large amounts of IFN-γ. IFN-γ, produced by the activated CD8+ cells and the NK cells, induces expression of major histocompatibility complex (MHC) class I molecules in the lower part of the follicular epithelium, resulting in presentation of follicular autoantigens to the CD8+ cells and loss of IP. IFN-γ also may induce MHC class II molecule expression by the follicular epithelium, leading to a second wave of CD4+ cells that may bolster CD8+ activity via released cytokines. Treg, regulatory T cell.