Chances are, as you're reading this, you've got an itch. (And if you don't now, you soon will.) It could be anywhere. Your chin, your scalp, your left elbow. Without thinking much about it, you reach out and (ahhh!) scratch it.

How fitting that one of life's most common irritants should be paired with one of its greatest satisfactions.

But while itching is a universal experience, doctors know little about it. While that may not seem like a big deal, it has great significance for the thousands who suffer from intractable pruritus (the $50-office-visit term for itching). For them, scratching brings no relief and probably makes matters worse. Medication, likewise, may be of little help; sufferers have few choices but to endure the maddening sensation.

That may soon change, thanks to research into the mechanism of itching and the development of more effective treatments. These studies also shed new light on the way nervous and immune systems interact.

Researchers have difficulty agreeing on exactly what constitutes itch. There's no synonym for the word, usually defined as an irritating sensa- tion that provokes the urge to scratch.

Some scientists hold that itching is a minor form of pain. They say the sensation is transmitted to the brain via the same neural pathways taken by pain signals.

Others, such as Dr. George Murphy, a dermatology and pathology professor at the University of Pennsylvania, believe itching, while related to pain, is in a class by itself.

"It's clearly a very separate physiological phenomenon," he says. "Our understanding of the physiology and pathology of it is embarrassingly rudimentary." Dr. Murphy notes that while people can easily feel pain inside their mouths, they seldom if ever experience itching there.

In a recent article in the journal Nature, Dr. Murphy and his research colleagues detailed their findings about a type of nerve fiber embedded in the skin that is found in close proximity to two types of immune cells, one of which is known to play a role in itching.

The team also found that nerve fibers release proteins called neuropeptides that stimulate the immune cells and, in turn, release chemicals that can lead to itching and other reactions.

"This has led us to believe there's a very intimate interplay between the nervous system and the immune system," Dr. Murphy says. "Conventional Western medicine and dermatology have not paid a tremendous amount of attention to the mind-body connection with respect to skin disease."

Still, Dr. Murphy cautions, "That's not to say we can necessarily liberate neuropeptides and consciously make ourselves itch at will."

Dr. Murphy says the research findings are suggestive in light of the number of itchy skin conditions, such as acne, eczema and hives, that often worsen when a person is under stress. Besides, some itching, such as that related to allergies, seems to have no other explanation.

"If I were going to bet money on trying to understand itching," he says, "I would start looking at these [mind-body] interconnections."

Many common itch-causing skin conditions are linked to

allergies, chemical sensitivities and immune system disorders, says Dr. Raoul Wolf, director of pediatric allergy and immunology at the University of Chicago Medical Center.

Hives, which can break out within seconds of eating a food to which one is allergic, are a good example. Toxins, from poison ivy or a mosquito bite, and infections, such as chicken pox, also commonly cause itching.

In each case, Dr. Wolf says, the itch is driven by mast cells.

These immune cells in the skin release histamines, chemicals that make skin tissue more permeable to other substances. The mast cells also secrete other chemicals that directly stimulate nerve endings, causing the itching sensation.

"The truth of it is that itch is really a modified pain sensation . . . it's a mild pain," Dr. Wolf maintains. Sunburn is a good example: A mild burn usually causes itchiness, he says, while a severe one is likely to be perceived as painful.

Dr. Wolf is somewhat skeptical of the nervous system-immune system connection as a cause of itchiness and believes stress exacerbates certain skin conditions because it alters a person's perception of what is usually a minor annoyance.

The scratch-itch cycle

And more scratching, which irritates the skin, only prompts more itchiness. The scratch-itch cycle eventually worsens the problem, he says.

Doctors usually treat allergy-caused itchiness with topical steroids, cortisone-like substances that reduce inflammation, or prescribe antihistamines, which block histamine receptor sites.

Illnesses that don't directly involve the skin, such as liver disease, gall bladder problems and thyroid abnormalities, are another major cause of itching. Experts believe failing organs may release toxins into the blood that stimulate nerve receptors in the skin.

In such cases, those affected may find little relief. Dan Torres Gates, an Albuquerque, N.M., painter, recalls that when his kidneys started to fail in 1986, he suffered severe itching on his ankles and shins. The problem worsened, he says, during

treatment with medication and dialysis.

"It was unbearable," says Mr. Gates, who has since had a kidney transplant. "You itch and itch and itch." He says he endured the torment by trying to put it out of his mind: "I'm a very strong-willed person."

Internal malignancy, especially lymphoma, may also first manifest itself as severe, unexplained itching, says Dr. Keyoumars Soltani, chief of dermatology at the University of Chicago Medical Center. A cancer screening is usually ordered for patients with such symptoms, he says.

Itching can also be caused by more mundane conditions, Dr. Soltani says. "One of the most important causes of itching, especially in the elderly, is dry skin," he says. Such patients should drink plenty of water and use skin lubricants to determine if that alleviates the problem.

While no medication blocks the itching sensation the way aspirin relieves a headache, new research suggests that tricyclic anti-depressants may relieve itching when applied to the skin, Dr. Soltani says.

Tricyclics, a class of drugs used since the 1950s to treat depression and anxiety, are usually taken orally. But the U.S. Food and Drug Administration is evaluating claims that the medication acts as a topical antihistamine.

For people suffering from itching caused by organ disease, ultraviolet light therapy has shown promise, though doctors aren't sure how it works.

Dr. Soltani offers some hope for victims of itching. With all the new research, he says, "I would bet in the next five years we will know a lot more about it."

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