Immunology of alopecia areata

Abstract

Alopecia areata is a condition that affects hair follicles and leads to hair loss ranging from small well-defined patches to complete loss of all body hair. Despite its high incidence, the pathobiology is not fully understood, and no single concept could be universally accepted. Alopecia areata is mostly considered to be an autoimmune disease, in which the collapse of hair follicle immune privilege plays a key role. Higher incidence rate in the female population and increased overall risk of other autoimmune disorders militate in favor of autoimmune hypothesis. Antibodies against multiple components of hair follicles almost exclusively attack in anagen phase, where melanogenesis takes place. It suggests involvement of melanogenesis-associated autoantigens as a target epitope. Some investigators believed that alopecia areata is not a truly autoimmune disease but is only 'consistent with' autoimmune mechanisms. High frequency of a positive family history up to 42% may reflects the contribution of heredity factors. In addition, no specific target autoantigen has been identified so far, and autoantibodies to hair follicle-associated antigens are detectable in normal individuals.

Keywords: alopecia areata; autoimmune; immunology.

Fig. 1

Effector CD8+T cells can be segregated into two populations on the basis of the cytokines they secrete similar to CD4+ T cells. Tc1 cells characteristically produce type-1 cytokines (IL-2, TNF-α, and IFN-γ), whereas Tc2 cells secrete type-2 cytokines (IL-4, IL-5, IL-6, IL-10, and IL-13). Tc1 cells as well as Th1 cells are involved in the protection against intracellular parasites and delayed-type hypersensitivity, but also, they play an important role in autoimmunity. Autoimmune diseases are characterized by increased Tc1/Tc2 ratio, in the same way as Th1/Th2 ratio

Fig. 2

Proposed pathogenesis of alopecia areata. The immunosuppressive milieu of the anagen hair bulb is modulated by immunosuppressive factors, such as α-MSH, TGF-β, and IGF-1, and suppressing NK-cells. Alopecia areata is hair follicle response pattern to various inflammatory events associated with e.g., IFN-γ-induced up-regulation of expression of MHC class Ia inside the hair follicle. Anagen-associated autoantigens are recognized, once they become exposed by ectopic MHC class I. That leads to autoimmune reactions against hair follicle