There has been extensive progress in understanding the cellular and molecular mechanisms of inflammation and immune regulation in allergic diseases of the skin and lungs during the last few years. Asthma and atopic dermatitis (AD) are typical diseases of type 2 immune responses. interleukin (IL)-25, IL-33, and thymic stromal lymphopoietin are essential cytokines of epithelial cells that are activated by allergens, pollutants, viruses, bacteria, and toxins that derive type 2 responses. Th2 cells and innate lymphoid cells (ILC) produce and secrete type 2 cytokines such as IL-4, IL-5, IL-9, and IL-13. IL-4 and IL-13 activate B cells to class-switch to IgE and also play a role in T-cell and eosinophil migration to allergic inflammatory tissues. IL-13 contributes to maturation, activation, nitric oxide production and differentiation of epithelia, production of mucus as well as smooth muscle contraction, and extracellular matrix generation. IL-4 and IL-13 open tight junction barrier and cause barrier leakiness in the skin and lungs. IL-5 acts on activation, recruitment, and survival of eosinophils. IL-9 contributes to general allergic phenotype by enhancing all of the aspects, such as IgE and eosinophilia. Type 2 ILC contribute to inflammation in AD and asthma by enhancing the activity of Th2 cells, eosinophils, and their cytokines. Currently, five biologics are licensed to suppress type 2 inflammation via IgE, IL-5 and | 1583 AKDIS et Al. 1 | INTRODUC TI ON Allergic diseases cause significant morbidity and mortality with almost one billion cases, accounting for a significant portion of overall healthcare costs. The dysregulated immune response, chronic inflammation, and remodeling in the affected tissues define a dynamic and heterogeneous spectrum of anaphylaxis, food allergy, asthma, allergic rhinitis, chronic rhinosinusitis with nasal polyposis (CRSwNP), and atopic dermatitis (AD). Two main subtypes of immune responses driving asthma and AD have been defined, namely type 2-high and type 2-low. 1-4 Precision medicine and biomarker discovery is important for the management of asthma and AD in the context of a better selection of good responders to treatment, prediction of outcomes, and design of disease-modifying strategies. Progress has been made in profiling the type 2 immune response-driven diseases. In contrast, the non-type 2 immune response in asthma and AD is insufficiently understood. The majority of patients with AD, CRS and asthma involve, or result from, an overexpression of type 2 inflammatory pathways (Figure 1). 5-7 A specific type 2 set of cytokines are produced during the induction and maintenance of allergic immune response with the contribution of epithelial cells, dendritic cells (DC), T cells, innate lymphoid cells (ILC), eosinophils, mast cells (MC), and basophils. Activation of Th2 and ILC2 pathways is at the core of type 2 inflammation. Th2 cytokines include interleukin (IL)-4, IL-5, IL-9, IL-13, and IL-31, and main type 2 cytokines of ILC2 are IL-5, IL-9, and IL-13. 8,9 IL-4 induces Th2 cell di...

Environmental exposure plays a major role in the development of allergic diseases.The exposome can be classified into internal (e.g., aging, hormones, and metabolic processes), specific external (e.g., chemical pollutants or lifestyle factors), and general external (e.g., broader socioeconomic and psychological contexts) domains, all of which are interrelated. All the factors we are exposed to, from the moment of conception to death, are part of the external exposome. Several hundreds of thousands of new chemicals have been introduced in modern life without our having a full understanding of their toxic health effects and ways to mitigate these effects.Climate change, air pollution, microplastics, tobacco smoke, changes and loss of biodiversity, alterations in dietary habits, and the microbiome due to modernization, urbanization, and globalization constitute our surrounding environment and external exposome. Some of these factors disrupt the epithelial barriers of the skin and mucosal surfaces, and these disruptions have been linked in the last few decades to the increasing prevalence and severity of allergic and inflammatory diseases such as atopic dermatitis, food allergy, allergic rhinitis, chronic rhinosinusitis, eosinophilic

We found for the first time that IL-4 and IL-13, signature type 2 cytokines, are able to induce periostin expression. We and others have subsequently shown that periostin is highly expressed in chronic inflammatory diseases-asthma, atopic dermatitis, eosinophilc chronic sinusitis/chronic rhinosinusitis with nasal polyp, and allergic conjunctivitis-and that periostin plays important roles in the pathogenesis of these diseases. The epithelial/mesenchymal interaction via periostin is important for the onset of allergic inflammation, in which periostin derived from fibroblasts acts on epithelial cells or fibroblasts, activating their NF-κB. Moreover, the immune cell/non-immune cell interaction via periostin may be also involved. Now the significance of periostin has been expanded into other inflammatory or fibrotic diseases such as scleroderma and pulmonary fibrosis. The cross-talk of periostin with TGF-β or pro-inflammatory cytokines is important for the underlying mechanism of these diseases. Because of its pathogenic importance and broad expression, diagnostics or therapeutic drugs can be potentially developed to target periostin as a means of treating these diseases.

The IL-13/periostin pathway induces IL-24 production in keratinocytes, playing an important role in barrier dysfunction in AD.