Abstract

Atopic dermatitis (AD) can be categorized into extrinsic and intrinsic types. The serum levels of IgE are high in extrinsic AD and normal in intrinsic AD. This dichotomy also corresponds to the following terminology: mixed AD vs pure AD, allergic AD vs non-allergic AD, and classical AD vs atopiform dermatitis. While extrinsic AD is the common type with high prevalence, intrinsic AD is approximately 20% in incidence and shows apparent female predominance. Extrinsic AD is closely associated with barrier perturbation and Th2-skewing immunological condition, but the causes and mechanisms of intrinsic AD remain elusive. In extrinsic AD, antigens can penetrate through disrupted barrier, and epidermal Langerhans cells serve as antigen-presenting cells to Th2 cells. In intrinsic AD, nonprotein antigens, such as metals and haptens, and Th1/Th17 cells participate as well as Th2 cells. Notably, intrinsic AD shows significantly higher percentages of positive patch test to nickel and cobalt than extrinsic AD, indicating high frequency of metal allergy in intrinsic AD.

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