Atopic dermatitis: Common extrinsic and enigmatic intrinsic types

Abstract

AD can be categorized into the IgE-high, extrinsic type and the IgE-normal, intrinsic types. They have also been called several different series of names: mixed AD versus pure AD, allergic AD versus nonallergic AD, and classical AD versus atopiform dermatitis. Although extrinsic AD is the classical type with high prevalence, the incidence of intrinsic AD is approximately 20%with female predominance. There have been a considerable number of findings regarding the barrier condition and immunological dysregulation in extrinsic AD, whereas the causes and mechanisms of intrinsic AD remain unclear. The skin barrier is perturbed in the extrinsic, and filaggrin gene mutation represents a typical cause of barrier impairment. In extrinsic AD, allergic conditions may be preceded by skin barrier impairment. Protein antigens can penetrate through the disrupted barrier, and epidermal Langerhans cells serve as antigen-presenting cells to Th2 cells. On the other hand, intrinsic AD is immunologically characterized by the higher expression of interferon-γ, and nonprotein antigens, such as metals and haptens, may induce dermatitis in intrinsic AD. In fact, intrinsic AD shows significantly higher percentages of positive reactions to nickel and cobalt than extrinsic AD.