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WASHINGTON (Reuters) - Acid reflux, a common condition that has been highly profitable for the makers of antacid drugs, may not be caused by stomach acid at all, U.S. researchers said on Thursday.Instead, gastroesophageal reflux disease or GERD might be caused by immune system cells causing inflammation, the team at the University of Texas Southwestern Medical Center said.Their study in rats showed that gastroesophageal reflux causes tissue in the esophagus to release immune chemicals called cytokines, which attract inflammatory cells. These cause the heartburn and chest pain that make GERD so distressing."Currently, we treat GERD by giving medications to prevent the stomach from making acid," said Dr. Rhonda Souza, who led the study published in the November issue of Gastroenterology."But if GERD is really an immune-mediated injury, maybe we should create medications that would prevent these cytokines from attracting inflammatory cells to the esophagus and starting the injury in the first place."Her team mimicked GERD in rats by connecting the duodenum to the esophagus, allowing stomach acid and bile to enter the esophagus. They assumed the stomach acid and bile would start to burn the esophagus right away."We were surprised to learn that it can take weeks for erosive esophagitis to appear after this operation," they wrote."That doesn't make sense if GERD is really the result of an acid burn, as we all were taught in medical school," said Dr. Stuart Spechler, a professor of internal medicine who led the study."Chemical injuries develop immediately. If you spill battery acid on your hand, you don't have to wait a month to see the damage."When they examined the tissue of the esophagus, they found it was filled with immune cells.More tests using human esophagus cells in lab dishes showed the cells sent out inflammatory signals with chemicals such as interleukin-8 when bile salts were put on them.About 40 percent of Americans suffer symptoms of GERD at some point, and 20 percent have it regularly, Souza said. It can eventually cause esophageal cancer.Treating GERD has helped turn drugs such as Prilosec and Nexium into mega-blockbusters.Prilosec, known generically as omeprazole, is now sold over the counter by Procter & Gamble Co, while the newer Nexium had $5.2 billion in sales in 2008.