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您请求的操作仅限于组中的用户:===皮肤作为免疫器官===皮肤通过物理屏障、生物分子、免疫和非免疫细胞复杂的网络和皮肤结构来保护宿主免受入侵====物理屏障====角质层中的角质层细胞有助于表皮的屏障功能。这些细胞以“砖块和灰泥”的方式排列，其间散布着神经酰胺、胆固醇和游离脂肪酸等脂质。*每个角质细胞含有一个与角蛋白丝束相连的脂质包膜，角蛋白丝束填满角质细胞的细胞内区室，从而增加其刚性。*角质层由三层组成，它既是一个由外而内的屏障，防止外来物质和微生物进入，又是一个由内而外的屏障，防止水分流失。 *表皮层中发现的连接粘附分子和紧密连接蛋白(Claudin-1/zonula occludin -1)也有助于物理屏障的形成。
这些成分的表达或功能紊乱可能导致不适当的屏障形成或皮肤疾病或皮肤炎症。研究表明，特应性皮炎患者皮肤中ZO-1和claudin-1的表达水平降低。==== Skin pH ==== Skin pH is acidic and ranges between 4 to 6 The body’s internal environment maintains a near-neutral pH (7–9). There is a gradient of 2–3 units between the SC and underlying epidermis and dermisAli SM, Yosipovitch G. Skin pH: from basic science to basic skin care. Acta dermato-venereologica. 2013 Mar 1;93(3):261-9.. Recent research suggests skin pH depends on several key enzymes involved in the synthesis and maintenance of a competent skin barrier. Age, anatomic site, sebum, sweat, genetic predisposition affect the pH along with the use of creams, soaps, and cosmetics. Various mechanisms maintain a low pH of the skin: * Enzymatic processes and fatty acids, sweat glands in the SC lower the pH of the skin. * Sweat glands secrete a vast collection of antimicrobial peptides, which restrain various microbes' growth on the skin. During rigorous physical exercise, dermcidin, an antimicrobial peptide, is secreted by the sweat glands onto the skin's epidermal surface. Research suggests that dermcidin gets activated in salty and slightly acidic sweat, which can perforate microbe membranes, allow water, and charged Zinc in sweat to gush across the cell membrane, kill the microbe. Wang E, Qiang X, Li J, Zhu S, Wang P. The in vitro immune-modulating properties of a sweat gland-derived anti-microbial peptide dermcidin. Shock (Augusta, Ga.). 2016 Jan;45(1):28. * Besides, the physiological pH of the skin is for commensal bacteria such as ''Staphylococcus epidermidis'', which helps in preventing pathogenic strains such as ''Staphylococcus aureus'' from establishing infections in the host. ==== Immune and Non-immune Cells ==== * Skin-resident immune cells promote tissue function in homeostasis and guard the body by actively sampling environmental antigens. Some resident immune cells migrate to lymph nodes to either induce peripheral tolerance to tissue self-antigens or initiate robust immune responses. In infections or tissue injury, immune cells resident in the skin and those infiltrating from the periphery interact to create an intricate defense network to resolve the insult and restore the tissue to its original state. * Skin-resident myeloid cells include Langerhans cells, dermal dendritic cells, macrophages, mast cells, and eosinophils that contribute to skin homeostasis by secreting growth factors needed for the survival of keratinocytes, fibroblasts, and endothelial cells. They phagocytose debris and apoptotic cells and support vasculature integrity. In inflammatory conditions, myeloid cells respond immediately and produce pro-inflammatory mediators that drive cell activation and infiltrate the affected area by peripheral immune cells. Skin myeloid cells also serve as a link between the innate and adaptive immune systems. ==== Biomolecules ==== Antimicrobial peptides (AMPs) and lipids are the main classes of biomolecules that participate in skin defense by disrupting bacterial membranes.

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