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Stress has profound effects on health, yet how it damages tissues remains poorly understood. Here, we show that acute stress triggers rapid hair loss and initiates autoimmunity. Under stress, hyperactivated sympathetic nerves release excessive norepinephrine, causing necrosis in rapidly dividing hair follicle transit-amplifying cells (HF-TACs) while sparing most hair follicle stem cells (HFSCs). This differential sensitivity stems from differences in cell death pathways, metabolic strategies, and calcium homeostasis, which render HF-TACs more susceptible to norepinephrine-induced calcium surges. HF-TAC necrosis releases cellular debris that triggers macrophage-mediated clearance and dendritic cell activation, ultimately leading to the activation and amplification of autoreactive T cells that can attack the hair follicle under inflammatory insults. Our findings reveal mechanistically how stress causes immediate tissue damage in highly proliferative HF-TACs via sympathetic nerve-induced necrosis, which in turn fuels the activation of autoreactive T cells capable of mounting future attacks against the same tissue.