Atopic dermatitis as Th2 disease revisited

T helper 1 (Th1) and T helper 2 (Th2) populations are a classical dichotomy to understand the immunological disorders, and this concept remains vital in atopic dermatitis (AD). While IL-4 and IL-13 are produced mainly by Th2 cells, mast cells, and basophils, these cytokines target these producers, indicating the presence of autocrine and paracrine stimulation. There are two types of receptors for IL-4 and IL-13, type I and type II. IL-4 binds to both type I and II receptors, while IL-13 has affinity to type II. Dupilumab is an IL-4Ra-antagonist that inhibits IL-4 and IL-13 signaling through blockade of the shared IL-4a subunit. The Th2-stimulating effect of thymic stromal lymphopoietin (TSLP) is triggered by its binding to TSLP receptor (TSLPR) on Th2 cells. The frequency of TSLPR+CD4+ T cells correlates with disease activity. CD4+ T cells directly interact with TSLP to produce a high amount of IL-4. Conversely, IL-4 induces TSLPR expression in T cells. Moreover, TSLP and IL-4 promote CCR4 expression, which is a chemokine receptor for CCL17/ TARC. Langerhans cells initiate epicutaneous sensitization with protein antigens and induce Th2 cell-mediated immune responses via TSLP signaling. Filaggrin deficiency, which cornified layer-damaged skin allows protein antigens to penetrate through the stratum corneum, leads to allergy where IL-4 and IL-13 are overproduced. Subsequently, IL-4 and IL-13 further depress filaggrin expression with vicious cycle. AD is generally a Th2 disease, but there exist several exceptional and undeniable facts to be considered, including Th1-activated intrinsic AD, Th1-infiltrating chronic AD skin lesions, and Th17 involvement.