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Q: Why is it so hard to get rid of psoriasis?

R.L., Wellesley

It’s not your fault. ‘‘There is no cure. Nothing can make psoriasis go away and make it stay away forever,’’ said Dr. David A. Jones, a dermatologist at Brigham and Women’s Hospital.

Psoriasis, a common, itchy skin disorder, is de- fined by a genetic predisposition of skin cells to react in a certain way to a variety of things: People can be sensitive to infections, alcohol, stress and certain drugs like beta-blockers or lithium, said Dr. Bernhard Ortel, a dermatologist at Massachusetts General Hospital.

In psoriasis, which affects 4 million to 5 million Americans, the immune system mistakenly causes skin cells to regenerate too quickly, according to the National Psoriasis Foundation (at www.psoriasis. org). Normally a skin cell matures and then falls off the body’s surface in 28 to 30 days, but a skin cell with psoriasis takes only three or four days to move to the surface. The extra skin cells pile up, forming red lesions with flaky, white scales that can itch, crack, bleed and sometimes be painful.

But there is a growing number of treatments, including Enbrel, which was approved recently and is already on the market for arthritis. Enbrel is one of a number of drugs in the relatively new category of ’’biologics,’’ which are engineered from proteins produced by living cells. Other biologics now approved for psoriasis include Amevive and Raptiva; two others, Remicade and Humira, are currently in clinical trials.

Another treatment, called PUVA (which uses ultraviolet A light and a drug called psoralen), pioneered at Mass. Gener, also helps by suppressing the immune response in the skin. For less-severe cases, topical steroid creams and ointments can help. Though psoriasis is rarely life-threatening, it can be life-wrecking for some people. According to a paper published this spring by Dr. Robert Stern, chief of dermatology at Beth Israel Deaconess Medical Center, more than 1 million Americans with relatively small areas of the body affected by psoriasis feel dissatisfied with their current treatment.