The skin has several different and important functions. It provides protection from both injury (such as abrasion) and dehydration. Since outer skin cells are dead and keratinized, the skin is waterproof, thereby preventing fluid (water) loss. The skin’s waterproofing also prevents water from entering the body when a person is immersed. The skin is a barrier against invasion by bacteria and viruses and is involved in the regulation of body temperature. It is the site for the synthesis of an inactive form of vitamin D. In addition, the skin contains receptors that receive the sensations of touch, vibration, pain, and temperature.