INTRODUCTION
The skin is one of the largest immunologic organs in the body and a continuous target for allergic and immunologic responses. Rising incidences of allergies have been reported worldwide. While the cause of this rise is not totally clear, it has been attributed to factors such as poor nutrition, stress, use of antibiotics, and growing up in clean urban homes while exposed externally to high air pollution.1-5 The skin barrier is the first interface between the environment and our immune system. This interface is constantly exposed to endogenous and exogenous factors including ultraviolet radiation, pollution, and damaging skincare products. Impairment of the skin barrier increases the likelihood of external antigens, irritants, and pathogens passing into the skin and driving inflammation, potentially leading to skin infections, allergies, and chronic inflammatory skin diseases such as atopic dermatitis (AD) and contact dermatitis (CD).6 This phenomenon has been referred to as “transcutaneous sensitizationâ€, and is highly dependent on skin barrier dysfunction.7
Skin Barrier Anatomy
Anatomically, the skin barrier can be divided into the epidermis and the dermis. The epidermis primarily consists
of keratinocytes arranged in several layers, with the stratum corneum (SC) at the top, a layer of cornified keratinocytes that physically prevents invaders from entering. The dermis contains collagen and elastin fibers, fibroblasts, proteoglycans, and nerve endings. Functionally, the skin barrier can be divided into four strata: the microbiome, chemical, physical, and immune layers (Figure 1). The microbiome layer consists of living microbial communities. The chemical layer includes natural moisturizing factors (NMF), human β-defensins, and the acid mantle, which maintains an acidic surface pH.8 Tight junctions and the SC constitute important parts of the physical layer, which also produces some of the compounds of the chemical layer. Sensing danger signals through pathogen- and damage-associated molecular patterns, resident immune cells of the immune layer work to clear invasions, repair the barrier, and maintain homeostasis. While each layer has unique functions, it also works interdependently in upholding overall integrity of the skin barrier.9
The Skin Microbiota and Dysbiosis
Like the gut microbiota, the healthy skin microbiota is fairly stable.10,11 It is populated by commensal organisms including bacteria, viruses, fungi, and mites, with the Staphylococcus, Cutibacterium, and Corynebacterium genus dominating. It is thought that commensal bacteria regulate potentially pathogenic species. As the outermost layer, microbial communities are first responders to changes in the environment and transmit signals to the immune system.9,12 Dysbiosis, or disruption of balance in the microbiome layer, has been extensively studied in the context of AD, the first
Skin Barrier Anatomy
Anatomically, the skin barrier can be divided into the epidermis and the dermis. The epidermis primarily consists
of keratinocytes arranged in several layers, with the stratum corneum (SC) at the top, a layer of cornified keratinocytes that physically prevents invaders from entering. The dermis contains collagen and elastin fibers, fibroblasts, proteoglycans, and nerve endings. Functionally, the skin barrier can be divided into four strata: the microbiome, chemical, physical, and immune layers (Figure 1). The microbiome layer consists of living microbial communities. The chemical layer includes natural moisturizing factors (NMF), human β-defensins, and the acid mantle, which maintains an acidic surface pH.8 Tight junctions and the SC constitute important parts of the physical layer, which also produces some of the compounds of the chemical layer. Sensing danger signals through pathogen- and damage-associated molecular patterns, resident immune cells of the immune layer work to clear invasions, repair the barrier, and maintain homeostasis. While each layer has unique functions, it also works interdependently in upholding overall integrity of the skin barrier.9
The Skin Microbiota and Dysbiosis
Like the gut microbiota, the healthy skin microbiota is fairly stable.10,11 It is populated by commensal organisms including bacteria, viruses, fungi, and mites, with the Staphylococcus, Cutibacterium, and Corynebacterium genus dominating. It is thought that commensal bacteria regulate potentially pathogenic species. As the outermost layer, microbial communities are first responders to changes in the environment and transmit signals to the immune system.9,12 Dysbiosis, or disruption of balance in the microbiome layer, has been extensively studied in the context of AD, the first
