CHICAGO â€"  Medical science efforts to harness the power of the immune system against cancer are beginning to bear fruit after decades of frustration, opening up a hopeful new front in the long battle against the disease.

In studies being presented Saturday, researchers said two experimental drugs by Bristol-Myers Squibb Co. significantly shrank tumors in some patients with advanced skin, lung and kidney cancers.

Especially promising was that the drugs worked against several types of cancer, researchers said of the early findings. Most of the patients whose tumors responded significantly to the treatment saw long-term results.

The new drugs empower the immune system to recognize cancer cells as an enemy and attack them, although the treatments can have serious side effects.

The immune system's ability to fight off the body's invaders has long been recognized against infectious disease. But cancer has largely escaped its reach and for decades fended off researchers' efforts to find a weakness. It turns out cancer shields itself by essentially putting the brakes on the human immune system.

Recent discoveries have led to advances in how to unleash the immune system, though one challenge has been to jump-start the body's defenses without injury to healthy, normal cells.

Researchers say the latest advances are beginning to trigger this process, and a host of other companies, including Roche Holding SA's Genentech unit, Merck, & Co. and GlaxoSmithKline  PLC, are also pursuing the so-called immunotherapies.

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