The skin is a layer of cells that cover most of the body and protect it from infection and water loss. The outer visible layer is made up of dead cells and is continually shed in small particles which eventually are the major component of dust. The inner layer of skin is living and is fed by capillaries which grow new cells to replace the outer layer.
Skin cells duplicate more often than any other type of differentiated cell and, as a result, are usually the first to show the signs of aging as the skin loses the ability to replace itself. The skin is particularly susceptible to the effects of ultraviolet radiation, which can cause premature aging of the skin.
All skin contains pores which allow the escape of sweat, which is used to help regulate body temperature. It also contains hair follicles, although the thickness of the hair also varies across the body.
Unlike most mammals, humans have very little hair covering their bodies. Only the top of the head and the pubic area have substantial amounts of thick hair, although the rest of the body is covered with follicles.
Skin also contains a substantial amount of pigment, although the amount of pigment varies both due to genetics and environmental factors such as exposure to the sun. The pigments protect the skin from damage from solar radiation, particularly ultraviolet rays.
The skin is also responsible for the production of Vitamin D, which is produced when the skin is exposed to sunlight.
The skin turns out to be an excellent habitat for bacteria, although most of the ones that do inhabit the skin are either harmless or beneficial. A typical human might at any given moment have about one thousand different species of bacteria in about 20 different biological families. However, about four different major families of bacteria make up the vast majority of bacteria that inhabit the skin.