LOS ANGELES (AP) _ Doctors say they have reversed cases of the severe bowel ailment Crohn's disease by giving patients doses of half-mouse, half-human antibodies.

About 400,000 to 500,000 Americans have Crohn's, a chronic inflammation of the small and large intestine that causes abdominal pain, diarrhea, fever and weight loss. Its cause is unknown.

In a study involving 108 patients, participants received single intravenous doses of a monoclonal antibody called cA2, which blocks tumor necrosis factor, a blood protein that plays a role in the inflammation.

Monoclonal antibodies are cells engineered to sweep through the blood and latch onto a target protein, in this case tumor necrosis factor. The cA2 is a ``humanized'' antibody _ part of it comes from a mouse, part of it from a human.

Of those who received the drug, 65 percent showed significant improvement after four weeks, compared with just 17 percent of those who received a dummy injection. Of the 65 percent who responded, half went into remission, and the rest showed dramatic improvement.

The benefits appeared as early as two weeks and persisted at least 12 weeks. No side effects appeared.

``These are people that don't respond to anything, people that have to be on steroids for years, which has a lot of side effects,'' said Dr. Stephan Targan, director of the Inflammatory Bowel Disease Center at Cedars-Sinai Medical Center in Los Angeles.

He reported the findings at a conference in San Francisco this week of four major gastrointestinal and liver disease associations.

Dr. Fabio Cominelli, head of the Division of Gastroenterology at University of Virginia Health Sciences Center in Charlottesville, said of the results: ``This is very exciting because what we have available right now for patients with Crohn's disease is not very good.''

But Cominelli cautioned that when the same antibody was used to treat rheumatoid arthritis, repeated doses were needed to avoid relapse.