The Many Functions of the Skin

Guy Heynen, MD
March 20, 2002 (Reviewed: March 7, 2004)

Skin as a multifunctional organ

In adults, the skin represents 16% of the total body weight, and is the largest organ of the body. Apart from its well-recognized social and cosmetic properties, skin has several other important purposes. It forms a barrier that protects us from the outside world, especially from infection and injury, and it prevents the loss of water and other body fluids. In addition to protecting us from the toxic effect of ultraviolet irradiation by producing melanin, when exposed to sun's rays it produces vitamin D3, an essential step in the formation of vitamin D that helps us build strong and well-shaped bones. The skin is extremely rich in nerve endings and sensory receptors, which are important for recognizing heat, cold, touch, and pain.

Skin as a temperature regulator

At normal outside temperatures, blood flows through the skin at rates 10 times higher than those necessary to feed the skin tissue. In hot climates skin blood flow can reach values up to 7 times higher than normal, while it can drop to an almost non-measurable rate in a very cold climate. In this way, the heat of the body is conducted to the skin where it is lost to the outside - through conduction, convection and radiation.

The skin also influences body temperature with the help of several million sweat glands distributed over the entire body. These glands excrete water, salt, lactic acid and waste protein products onto the skin surface, not unlike the way the kidney works. Once on the skin, the sweat evaporates, cooling the surface and the blood flowing beneath it.

Skin as a protective border

The skin is also part of one's natural resistance (also called natural immunity) against invasion by micro-organisms. The dryness and constant scaling of the skin, fatty acids in the sebum, and lactic acid in the sweat, represent natural defense mechanisms against invasion by micro-organisms. But there is more. Normal skin contains a surveillance system under a central control unit responsible for defense against foreign intruders. This system is generally known as the immune system, and for the immune system, "foreign" means what is "not-self". The exact behavior of one's immune system is therefore deeply rooted in one's genes, and can show important variations between individuals; this explains how some people, but not others, develop atopic dermatitis.

What goes wrong in atopic dermatitis?

Two main types of cells need to be considered to understand what is going awry in dermatitis. First, skin cells specialized in the detection and processing of foreign material, known as Langerhans cells, are distributed throughout the epidermis. These cells have the ability to pass on foreign material by 'presenting' them to other specialized immune cells (lymphocytes) that come via the blood vessels from the central control unit of the immune system, the thymus, and have established their home in the dermis. They are called T-cells or T-lymphocytes (T stands for thymus).

Langerhans cells also communicate with neighboring cells of the epidermis, the keratinocytes. Exchange of information between all these cells takes place with the help of chemical substances, called cytokines. There are close to 20 different cytokines known today, each bringing a specific set of messages to other cells in order to either activate or inhibit their numbers and activity. Excess production of some cytokines and increase in the number of T-cells are usually associated with recognition of a foreign invasion and set up the machinery to hunt, capture and eliminate the aggressor.

Aggressors can have many different faces -- bacteria, viruses, parasites, foreign animal or plant-derived material such as egg or milk, or droppings of house-dust mites - and, in addition to any immune response reactions, can produce inflammatory reactions of varying severity. In the case of aggression by bacteria, elimination of the offending intruders is generally successfully achieved by the inflammatory reaction. But in the case of intrusion of a foreign protein such as house-dust mites, only some persons will mount an immune response. In such cases, decreased exposure, by regularly vacuuming, for instance, is a better approach, since house dust mite is not a nuisance (i.e. perceived as foreign) for most people.

Conclusions

Skin, therefore, is mainly a protective boundary with a sophisticated network system that provides intelligence to the body to protect it against environmental aggressors and insults to its integrity. It is equipped to deal with the aggressors; the activation or proper function of this machinery is essential to keep out or eliminate "non-self" intruding biological material. Exaggerated activation of this machinery is what occurs in atopic dermatitis and allergic contact dermatitis. Unfortunately, however, identification of the cause of this abnormal activation is not possible in all cases.

Source

• Atopic dermatitis: new insights and opportunities for therapeutic intervention. DY. Leung, J Allergy Clin Immunol, 2000, vol. 105, pp. 860--876

Related Links
The Structure of the Skin
What is Dermatitis?
What Causes Dermatitis?

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