The skin is an important and largest organ of the human body, with special independent functions. In adults, the skin covers an area of about 1.5-2.0 square meters and weighs about 5% of body weight; if the subcutaneous tissue is included, the total weight can reach 16% of body weight. The skin consists of the epidermis, dermis and subcutaneous tissues. At the most surface is the epidermis, only the epidermis is injured will not bleed, because the epidermis does not have blood vessels. The thin epidermis is also divided into five layers, from the outside to the inside are the stratum corneum, the hyaline layer, the granular layer, the spiny layer and the basal layer. After the metabolism of the stratum corneum is complete, it becomes scaly and falls off. You can not underestimate the stratum corneum, if it is injured skin barrier role can be weakened. Under normal circumstances, the human epidermal cells generally have a growth and shedding cycle of 28 days, but in some skin diseases it can be greatly shortened, for example, in psoriasis the epidermal growth and shedding cycle can be shortened to 3-4 days, so there will be a lot of scales shed. The skin has important physiological functions, including the role of protection against various external stimuli and injuries, the role of regulating body temperature, the role of secretion and excretion, the role of absorption, the role of sensation, and the role of participation in body metabolism and immunity. It is important to note that the skin is an important immune organ and a unique immune unit. Therefore, once there are external factors or changes in the body’s internal functions, various types of allergic reactions (i.e. allergic reactions) and immune skin diseases can occur.