Immune Cells in Hair Follicle Physiology and Pathology

The hair follicle (HF) is a sophisticated mini-organ with intrinsic cyclic activity that generates hair shafts. A variety of immune cells, including macrophages, mast cells, T cells, and Langerhans cells, reside around HFs, collectively forming a delicate and intricate immune microenvironment. This review systematically summarizes the specific distribution, dynamic alterations, and biological functions of diverse immune cells within the HF microenvironment, and highlights their dual roles in maintaining HF physiological homeostasis and mediating pathological damage. Under physiological conditions, these immune cells maintain normal HF physiological functions by precisely regulating hair follicle stem cell (HFSC) activity, modulating hair cycle progression, and defending against microbial invasion. However, multiple stressors, including infection, mechanical trauma, and psychological stress, can disrupt HF immune homeostasis and consequently trigger pathological dysfunctions. This article further expounds the pivotal roles of specific immune cells in HF-associated disorders. An in-depth understanding of the mechanisms underlying physiological regulation and pathological imbalance of the HF immune system not only provides novel mechanistic insights for elucidating the pathogenesis of alopecia but also lays a solid theoretical foundation for developing innovative therapeutic strategies targeting immune modulation.