It's the cancer no one worries about because it's so common and so curable. A little patch on the nose or the back of the hand. A quick trip to the dermatologist will get rid of the malignant spot.

President Reagan, who had a cancerous lesion removed from his nose last week, is only one of the 500,000 Americans who will be treated for some form of skin cancer this year. Most skin cancers are related to sun exposure, but what is worrisome is that the number of cases is getting larger.

Except for lung cancer, virtually all the other cancers have stabilized or decreased in numbers. What's more, skin lesions are no longer confined mainly to people over 40 or those in outdoor occupations.

"We are seeing younger and younger people with skin cancer -- ages 18, 20, 25," said Dr. Stephen Katz, chief of dermatology at the National Cancer Institute. "We are suffering the consequences of past enjoyment."

The rise in skin cancer comes at a time when medical researchers have dramatically changed their view of the role of the skin in protecting the body from disease. Far from being a passive covering of the body, the skin is now seen as a complex organ that plays a key role in the immune system. Who's at Risk

Most of the concern over skin cancer is centered on the deadliest form of the disease, malignant melanoma, which strikes some 26,000 Americans a year and has been going up 3.4 percent every year for more than a decade.

Basal cell carcinoma, which President Reagan had, is the most common form of skin cancer -- and one of the least threatening. Another similar form is squamous cell carcinoma. Left untreated, these tumors may over a period of years destroy tissue in the local area and even invade the underlying bone, but they almost never spread to distant parts of the body.

The Skin Cancer Foundation in New York estimates that the incidence of skin cancer has increased by 15 to 20 percent since 1981.Health officials also say that many cases of non-melanoma cancers go unreported.

The dominant factor in skin cancer -- though not the only one -- is exposure to sunlight. Ultraviolet rays, which make up about 5 percent of sunlight, do the damage.

Most cases today of skin cancer are the result of longterm exposures that began up to 30 years ago, when the pursuit of the golden tan was in high fashion and people started to have more leisure time. As a result, a large sunburn generation has emerged in the United States that is now going to the doctor because of skin lesions.

Another factor in the rise in skin cancer may be a weakening of the protective ozone layer in the stratosphere, which like a cosmic parasol prevents harmful sun rays from reaching sea level. Just how much the ozone layer has been weakened -- or if the damage has been significant -- is a matter of scientific debate. Most researchers agree that a major breakdown of the ozone layer would have disastrous consequences to human health and would damage food production and cause other ecological crises.

"If our ozone layer is depleted," said Dr. Darrell Rigel, a cancer researcher at New York University, who testified before Rep. Henry A. Waxman (D-Calif.) and his House subcommittee on health and the environment, "one could envision a future situation where despite maximal sun avoidance and screening, people in their everyday activities alone might receive enough radiation to develop skin cancer."

Rigel has predicted that the average person's risk of contracting melanoma in his or her lifetime has risen from an estimated 1982 level of one in 250 to one in 135 today and will probably be one in 90 by the end of the century.

Population studies show that people with light skin living in latitudes near the equator have higher rates of skin cancer than those who live in more northern climates. Blacks, who are protected by melanin, the dark pigment found in the deep layers of the skin, rarely develop skin cancer.

Genetics, too, probably plays a role -- largely in determining the type of complexion. Studies show that fair-skinned, blue-eyed redheads with a tendency to freckle are at greatest risk. Others with light skin, light hair and especially light eyes also are at high risk.

Patients with the rare and frequently fatal genetic disorder xeroderma pigmentosum have inherited a hypersensitivity to the cancer-causing effect of ultraviolet light. Their skin cells are no longer capable of repairing the genetic damage caused by sunlight, so the biological process that leads to cancer is accelerated.

But medical science has left many questions unanswered. Why, for example, do many people with fair skin who spend all their lives in the sun not get skin cancer? Why do others with little exposure in the sun develop a malignancy of the skin? Can a cancer be prevented from recurring?

To measure the impact of sun exposure on skin cancer, researchers at Johns Hopkins Medical Institutions are following 800 Maryland watermen on the Eastern Shore. In this study directed Dr. Edward A. Emmett, chief of the Center for Occupational Health, and Dr. Hugh Taylor, associate professor of ophthalmology, each person is classified according to complexion and general health status. Radiation monitors are then placed on the hats of the fishermen as they stream out from Smith's Island, Hooper's Island and Crisfield each day. Most fishermen start work at 4 or 5 a.m. and come back into port around 1 or 2 p.m.

"Fishing has not changed, in many important aspects, for centuries," said Emmett. "By law, watermen can only do certain things at certain times -- there's a season for crabbing, a season for oystering -- so we can reconstruct ultraviolet exposure over a lifetime" for each man in the study.

Emmett's group is now analyzing data, which has been collected since 1984, to determine precisely how much sun is required for certain complexions to develop skin cancer. He expects preliminary results in the next few months.

One study has shown that a very severe sunburn in childhood can increase the risk of developing melanoma later in life, regardless of how much time is subsequently spent in the sun. For the most common skin cancers, however -- basal cell and squamous cell carcinomas -- chronic exposure to sunlight is thought to be the risk factor that counts.

Tool of the Immune System

Until fairly recently, the skin was a hot subject only in cosmetic laboratories, where the search for the fountain of youth has translated into a booming cosmetic industry.

In the last decade, with increasing evidence that the skin has a major effect on the body's immune system, the field of dermatology has exploded.

New studies suggest that exposure to sunlight may reduce a person's ability to respond to infection -- at least for a brief period. In a study published in 1983 in the British journal Lancet, Australian scientists tested volunteers who were exposed to ultaviolet radiation in a tanning parlor for two weeks. They were then exposed to a chemical similar to poison ivy. Results showed that their ability to generate an immune response was diminished significantly.

Such results show that the skin is a far more complex organ than previously thought. The outermost layer of skin -- the epidemermis, which stretches from the scalp to the soles of the feet -- is about as thin as plastic wrap. Within the epidermis are at least four different populations of cells. About 90 percent are called keratinizing cells, which are involved in the body's production of interferon, a substance that can boost the immune system and fight viral infections, and interleukin-1, which activates white blood cells, the body's foot soldiers in fighting disease.

In recent years, attention has focused on a rare group of cells in the epidermis -- the Langerhans' cells. Named after German medical researcher Theodor Langerhans, who described the cells in the 1800s, these cells seem to play a key role in the immune system.

In patients with acquired immune deficiency syndrome, for example, who are at risk of developing the rare skin cancer of Kaposi's sarcoma, researchers have found that the Langerhans' cells decrease in number and intensity. They suspect that these cells can be infected by the AIDS virus.

Other pieces of the skin-immune system puzzle come from people who have weakened immune systems because of disease or certain drugs they must take.

Kidney transplant patients, for example, depend on a lifetime maintenance dose of the drug cyclosporin, which supresses the immune system so that it won't reject the transplanted organ. Studies show that kidney transplant patients have a higher rate of skin cancer compared with the general population.

"It's all coming together now: the skin is an immune organ," said Katz of NCI. "We are looking at skin in a whole new way."

Skin: Model for Other Cancers

The common basal cell and squamous skin cancers are among the most curable of all cancers. If caught early -- as it was in Reagan's case -- the lesion can be easily removed.

Patients with melanoma that has not spread to other parts of the body also have a good outlook. Nearly 90 percent of patients live at least five years after diagnosis and are considered cured, according to the American Cancer Society. The problem is that melanoma spreads very rapidly, so early detection is critical.

Standard therapy for a non-melanoma lesion on the surface of the skin involves a minor surgical procedure in which a curved blade, or curette, is used to scoop out the relatively soft tumor, which differs in texture from the firmer healthy skin tissue.

A more precise type of surgery known as the Mohs' technique was used on Reagan. In this procedure, surgeons shave off a thin layer containing the tumor, which is then examined under a microscope for cancer cells. The shaving process is repeated until doctors are satisfied that no malignant cells remain in the patient.

Because this technique costs two to three times more than the traditional method, it is recommended in only three circumstances: when the tumor is relatively large and poorly defined, when it occurs near scar tissue or when it is a morphea-like tumor -- a type that has an aggressive growth pattern and is firmer than other skin cancers, making it more difficult to distinguish from healthy tissue.

"My guess," said Dr. Mervyn Elgart, chief of dermatology at the George Washington School of Medicine, "is that Reagan's surgeons felt the new lesion was close enough to the old scar, or was a morphea-like tumor, so that they didn't feel confident" using the standard technique that had been used to remove Reagan's two previous skin cancers on July 30, 1985, and Oct. 10, 1985.

Most skin cancers occur, like Reagan's, on the head and neck, and 40 percent are on the nose, said Dr. Neil Swanson, director of cutaneous surgery and oncology at the University of Michigan Medical School. "One of the biggest causes," said Swanson, "is the reflection of sunlight off glasses onto the nose."

For malignant melanoma, surgeons cut out the main tumor and may also remove nearby lymph nodes to search for evidence that the cancer has spread.

Scientists are also experimenting with immunological treatments for skin cancer. In a study by Drs. Hugh Greenway and Roger Cornell at the the Scripps Clinic and Research Foundation in La Jolla, Calif., eight patients received injections of alpha-2-interferon into basal cell carcinoma tumors on their neck and back three times a week for three weeks. Two months later, biopsies showed that all of the cancer cells had disappeared.

A long-term study is now under way to confirm the initial results, Greenway said.

Although there were some flu-like side effects, Greenway said, the interferon treatments could soon replace surgery as the main treatment for basal cell cancers, especially for tumors in sensitive locations such as the face.

Other compounds are also being investigated as a way to prevent skin cancer in high-risk people. In a nationwide study sponsored by NCI, researchers are testing retinoids -- a vitamin A compound -- on patients who have had two or more basal cell cancers in the last two years. Such patients have a 50 percent chance of a recurrence within two years. Preliminary findings are expected in October, but final results will not be ready for several years.

Because it is much easier to monitor the effects of experimental agents on the skin than on other organs, scientists are testing a range of immunological therapies in the hopes of devising new treatments for other types of cancer.

"Melanoma may be a model system," said Dr. Arthur Sober, associate chief of dermatology at the Massachusetts General Hospital in Boston. "If we can understand melanoma, we will be light years ahead in understanding stomach cancer, lung cancer and other cancers."

Washington Post staff writers Robin Marantz Henig and Larry Thompson contributed to this article.