Fig. 2.

Hair growth cycle. The normal hair growth cycle consists of the anagen (active growth) phase, the catagen (controlled apoptosis) phase, the telogen (resting) phase, and the return to anagen (also called exogen) phase. In the anagen phase, the hair follicle is attached to the dermal papilla and able to receive nourishment from the blood vessels that run through the papilla. The hair is actively growing upwards out of the scalp. Anagen lasts about 2–7 years, and 88–90% of hairs on the head are in anagen at any given time. During catagen, the hair follicle begins to separate from the dermal papilla due to controlled apoptosis of epithelial cells. Catagen lasts about 2–4 weeks and 2% of hairs are in catagen at any given time. During telogen, the follicle detaches from the dermal papilla and thus its source of nutrients. Without its nutrient source, the hair dies and falls out of the follicle (indicated with the black arrow). The follicle is in its resting stage. Telogen lasts about 2–4 months and 8–10% of hair is in telogen at any given time. As the cycle returns to the anagen phase, the dermal papilla reunite with the hair follicle and the hair matrix begins to form a new hair. In AA, the normal hair growth cycle is disrupted (indicated with red text). Immune privilege is lost during the anagen phase due to increased MHC I activity and decreased CD200 immunoregulatory presence. The hair follicle is prematurely launched into the telogen phase with increased inflammation and destruction during the catagen phase. Created with Procreate.