The epidermis is the outermost layer of our skin. It protects us from external harmfull elements (UV, tobacco, pollution, bacteria) and controls permeability (hydration levels). It is an epithelial tissue with three types of cells. Keratinocytes make up 90% of the cells. They synthesize keratin (also present in hair), which protects the epidermis and its permeability. The other two types of cells are melanocytes (skin pigmentation cells which protect from UV) and Langerhans cells, which form part of our immune system. The epidermis itself is divided into five layers: the stratum corneum, the clear layer, the granular layer, the spinous layer and the deep basal layer. Epidermal cells are produced in the basal layer, from where they migrate and transform and specialize as they cross the other layers. On the surface of the epidermis (the skin that we 'see') lies a thin film consisting of lipids, fat and perspiration - the hydrolipidic film. This film fortifies helps in the protection of the skin, defending it from bacteria and preventing the epidermis from drying out. It's on this upper level that active ingredients and nourishing treatments work their wonders.