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October 06, 1991|FREDERIC GOLDEN | Golden writes about medicine and science from San Francisco.

During tanning, the skin produces an above-normal amount of melanin, a protective pigment that accounts for the skin's coloration. Unfortunately, as studies by Dr. D. Martin Carter of Rockefeller University have shown, by the time the pigment reaches the skin's surface, where melanin can absorb the sun's harmful rays, the skin already has been damaged--as indicated by redness.

"People who like to tan simply refuse to take sunburn seriously," Carter says. "The redness represents injury--damage that will accumulate over the years. This is why skin cancer is the most common of malignancies." And it's why dermatologists are almost uniformly opposed to tanning parlors. They strongly recommend the use of sunscreens--especially between the hours of 10 a.m. and 2 p.m., when the sun is at its highest and the UVB component of its light at its strongest. "Sunscreens are another important advance," Wintroub says. "They do a terrific job"--along with sunglasses, hats and other protective clothing.

Besides being blamed for a documented increase in skin cancer cases in recent years, excessive sunlight is a prime culprit in the development of the dry, leathery look known technically as photoaging and of the stigmata--"liver spots"--on the faces and hands of older people. In addition, too much sun accelerates the development of cataracts and can provoke allergic reactions to certain chemicals.

Sunburns also may have more sweeping effects on the body. In research at the University of Texas Southwestern Medical Center in Dallas, dermatologists have found that UVB not only causes direct damage to the skin but also impairs the immune response. Dr. Paul Bergstrasser, the lead researcher in the field, says: "I think that the skin not only provides immune protection for its own tissue but plays a role--a role we don't understand very well--in protecting the whole body."

Tantalizing clues as to how this defensive mechanism functions with the rest of the body have been uncovered. Researchers have found that the skin's so-called Langerhans cells are able to seize invading microorganisms while calling upon the help of white blood cells known as T-cells to destroy them.

Other skin cells that work with the immune system--and may be considered part of it--are keratinocytes. As their name implies, they produce keratin, a family of tough, waterproof proteins that coat the body and are the basic component of hair and nails. But that's apparently not all they do: Scientists have found that these cells produce a hormone that stimulates the growth and development of T-cells--the immune system cells that are the special target of the AIDS virus. Before this discovery, the production of T-cells was regarded as the exclusive responsibility of the thymus, a gland at the base of the neck that is crucial to the development of the immune system early in life. (Doctors have also found that the AIDS virus can lie dormant in Langerhans cells, which originate in the bone marrow and migrate to the skin, before any overt symptom of the disease appears.)

The surprisingly versatile keratinocytes also produce a substance called interleukin-1, which in turn stimulates the T-cells into making a related substance called interleukin-2--a material that triggers the manufacture of special T-cells called helper cells. One of the helper cells' critical jobs is to mobilize still other components of the immune system to start churning out antibodies--proteins specifically designed by the body to latch onto the outer surfaces of invading materials and neutralize or destroy them.

Researchers are discovering that the skin's chemical defenses deal with invading substances directly. When certain carcinogens enter the skin, they are attacked quickly by skin enzymes that render the invaders harmless. But not always; sometimes the skin's enzymes hook up with ordinarily inoffensive substances, such as benzopyrene--a byproduct of the incomplete burning of heavy oil or coal--and turn them into carcinogens. In its transformed lethal state, benzopyrene is able to enter the nuclei of cells, damage their DNA and make them cancerous.

To counter such effects, scientists are exploring the enhancement of those enzymes that might offer protection against intruding chemicals and the weakening of those that increase the potency of the invaders. Dr. David R. Bickers of Case Western Reserve University in Cleveland says that it's theoretically possible to create lotions that could increase the production of enzymes that would disable invading chemicals.

The chemical transformations wrought naturally by the skin are already proving helpful, as illustrated by the skin's reaction to the steroid cortisone, often used to treat injuries. A relatively weak anti-inflammatory agent on its own, it works as a healing agent largely because chemical agents in the skin convert it into much more effective hydrocortisone.

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