The natural barrier of the skin is a “brick wall structure” consisting of “lipids” and “natural moisturizing factors” between the cells of the stratum corneum and the cells, with a The “sebaceous membrane”, which together form a natural protective barrier for the body. This is the famous “brick wall theory” proposed by Peter in 1983. Sebaceous membrane: It is composed of “oil” secreted by sebaceous glands (the main component is squalene), “water” secreted by sweat glands, and “natural moisturizing factor” secreted by epidermis. Brick wall structure: The epidermal stratum corneum cells are like “bricks”, while the “lipids” (mainly ceramides) and “natural moisturizing factors” between the stratum corneum cells are like “mortar”. The “lipids” (mainly ceramides) and “natural moisturizing factors” between the stratum corneum cells are like “mortar” that keep the stratum corneum cells together. From the above, we can see that the lipids in the sebaceous membrane on the surface of the skin and the lipids between the stratum corneum are different in origin and composition. The former is derived from sebaceous gland secretions and its main component is squalene, while the latter is derived from keratin-forming cells and its main component is ceramide. The barrier of the epidermis is very important for human health and its function is divided into two aspects: first, to prevent and keep the human skin from losing water, i.e. moisturizing function (of course, it also prevents external moisture from entering the body easily); second, to prevent bacteria, fungi and viruses on the skin surface from entering the body and causing diseases. Therefore, when the skin is intact, Langerhans cells recognize skin commensal bacteria without causing an immune response; however, after the skin is broken, bacteria, fungi and viruses, which are not normally pathogenic on the skin surface, enter the dermis and are recognized by dendritic cells, deliver commensal antigens, activate the immune system and induce inflammation. Therefore, when the skin barrier function is disrupted, it not only decreases the moisturizing function of the skin; at the same time, it can lead to many immune-related skin diseases, such as eczema, atopic dermatitis, allergic dermatitis, and can also make eczema, facial dermatitis, psoriasis and other skin diseases aggravated. This establishes a theoretical basis for how skin breakage (e.g., caused by scratching) and infection can lead to the occurrence of inflammatory skin diseases, and how patients with inflammatory skin diseases can prevent and treat skin diseases by protecting the skin barrier, by avoiding infection and by avoiding skin breakage. At present, the research on skin barrier function has become a new hot topic.