White hair in alopecia areata: Clinical forms and proposed physiopathologic mechanisms

Alopecia areata (AA) is a common form of nonscarring hair loss. It is believed to be a consequence of an immune-mediated stimulus, probably involving autoreactive T cells against antigens present in the hair follicle. The exact antigen is still unknown; however, some authors have proposed that melanogenesis-associated molecules might trigger autoimmunity. Although transient white hair regrowth is a common and well-known situation in AA, there are other types of white hair phenomena in this context, including permanent white hair regrowth, sparing of white hair in a patchy pattern, or sparing in a diffuse pattern, giving the appearance of the so-called overnight graying phenomena or canitis subita. In this review, we aim to describe the different clinical aspects of white hair in AA, as well as the proposed pathophysiologic mechanisms involved in this phenomena.