The skin is the outer covering of the body. In humans, it is the largest organ of the integumentary system made up of multiple layers of ectodermic tissue, and guards the underlying muscles, bones, ligaments and internal organs.  Human skin is not unlike that of most other mammals except that it is not protected by a pelt and appears hairless though in fact nearly all human skin is covered with hair follicles. There are two general types of skin, hairy and glabrous skin.  The adjective cutaneous literally means "of the skin" (from Latin cutis, skin).
Because it interfaces with the environment, skin plays a key role in protecting (the body) against pathogens  and excessive water loss. Its other functions are insulation, temperature regulation, sensation, synthesis of vitamin D, and the protection of vitamin B foliates. Severely damaged skin will try to heal by forming scar tissue. This is often discolored and de-pigmented.
In humans, skin pigmentation varies among populations, and skin type can range from dry to oily.
Skin can be classified based on its reaction to ultraviolet radiation: 
Type Definition Description
I Always burns, never tans Pale, Fair, Freckles
II Usually burns, sometimes tans Fair
III May burn, usually tans Light Brown
IV Rarely burns, always tans Olive brown
V Moderate constitutional pigmentation Brown
VI Marked constitutional pigmentation Black
Hygiene and skin care
The skin supports its own ecosystems of microorganisms, including yeasts and bacteria, which cannot be removed by any amount of cleaning. Estimates place the number of individual bacteria on the surface of one square inch (6.5 square cm) of human skin at 50 million, though this figure varies greatly over the average 20 square feet (1.9 m2) of human skin. Oily surfaces, such as the face, may contain over 500 million bacteria per square inch (6.5 cm²). Despite these vast quantities, all of the bacteria found on the skin's surface would fit into a volume the size of a pea.  In general, the microorganisms keep one another in check and are part of a healthy skin. When the balance is disturbed, there
may be an overgrowth and infection, such as when antibiotics kill microbes, resulting in an overgrowth of yeast. The skin is continuous with the inner epithelial lining of the body at the orifices, each of which supports its own complement of microbes. Proper skin hygiene is important because unclean skin favors the development of pathogenic organisms. The dead cells that continually slough off the epidermis mix with the secretions of the sweat and sebaceous glands and the dust found on the skin form a filthy layer on its surface. If not washed away, the slurry of sweat and sebaceous secretions mixed with dirt and dead skin is decomposed by bacterial flora, producing a foul smell. Functions of the skin are disturbed when it is excessively dirty; it becomes more easily damaged, the release of antibacterial compounds decreases and dirty skin is more prone to develop infections.
Cosmetics should be used carefully on the skin because these may cause allergic reactions. Each season requires suitable clothing in order to facilitate the evaporation of the sweat. Sunlight, water and air play an important role in keeping the skin healthy.
1. "Skin care" (analysis), Health-Cares.net, 2007, webpage:
2. Marks, James G; Miller, Jeffery (2006). Lookingbill and Marks' Principles of Dermatology. (4th ed.). Elsevier Inc.
3. Proksch E, Brandner JM, Jensen JM. (2008).The skin: an indispensable barrier. Exp Dermatol. 17(12):1063-72.
4. Madison KC. (2003). Barrier function of the skin: "la raison d'être" of the epidermis
5. 8. Theodor Rosebury. Life on Man: Secker & Warburg, 1969 ISBN 0-670-42793-4
6. 13. Weller, Richard; John Hunter, John Savin, Mark Dahl (2008). Clinical Dermatology (4th ed.). Malden, Massachusetts, USA: Blackwell Publishing. pp. 268. ISBN 978-1-4051-4663-0.
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