Alopecia areata: autoimmune basis of hair loss.

Alopecia areata (AA) is a heterogeneous disease characterized by nonscarring hair loss on the scalp or any hair-bearing surface. A wide range of clinical presentations can occur -- from a single patch of hair loss to complete loss of hair on the scalp (alopecia totalis) or the entire body (alopecia universalis). Particularly in severe or chronic cases, AA may cause considerable psychological and emotional distress for affected individuals. The estimated lifetime risk of developing AA is 1.7%. While the precise etiology of this common disorder has not been elucidated, a substantial body of evidence suggests that AA is an organ-specific, autoimmune disease, targeted to hair follicles. However, the antigenic target(s), mechanisms, and consequences of autoimmune attack in AA have yet to be determined. Here, we critically explore the evidence supporting the hypothesis that AA is an autoimmune disease and propose specific pathways by which self-directed immune responses are generated.