The hair follicle maintains a partial "immune-free zone" around its growing cells, protecting the genetically distinct hair shaft from immune attack.The hair shaft is antigenically distinct from the host and would normally trigger immune responses. The follicle suppresses local immune activity via IL-10, TGF-β, and MHC class I downregulation to prevent this. In alopecia areata, this immune privilege collapses — for reasons not fully understood — allowing cytotoxic T cells to infiltrate the follicle matrix, producing the characteristic patchy hair loss. Thymulin’s proposed mechanism for hair loss partly involves restoration of follicle immune privilege via immunomodulatory signaling.