2.1.1　Skin barrier

The skin directly interfaces with the external environment，the most important function of which is to provide a protective barrier against the outside world，protecting organisms from environmental dangers such as extreme temperatures，ultraviolet，allergens，toxins，and microbes.Impaired barrier function has been linked to dry skin，itching，peeling，and exudation，and is closely related to the occurrence of skin disease.(https://www.daowen.com)