WASHINGTON – An ordinary cup of tea may be a powerful infection fighter, a study suggests. Researchers report in the Proceedings of the National Academy of Sciences that they have found in tea a chemical that boosts the body’s defense fivefold against disease.

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They said the chemical primes immune system cells to attack bacteria, viruses and fungi and could, perhaps, be turned into a disease-fighting drug someday.

Dr. Jack F. Bukowski of Brigham and Women’s Hospital in Boston and Harvard Medical School said Monday that he and his co-authors isolated the chemical in the laboratory and then proved with a group of volunteers that it did protect against germs.

“We worked out the molecular aspects of this tea component in the test tube and then tested it on a small number of people to see if it actually worked in human beings,” said Bukowski. The results, he said, gave clear proof that five cups of tea a day sharpened the body’s defenses against disease.

Penny Kris-Etherton, a nutrition specialist at Pennsylvania State University, said Bukowski’s study adds to a growing body of evidence that tea is an effective disease fighter.

“This is potentially a very significant finding,” she said. “We’re seeing multiple benefits from tea.”

But she said the work needs to be confirmed in a much larger study, involving more people.

In the study, Bukowski and his co-authors isolated from ordinary black tea a substance called L-theanine. He said the substance is found as well in green and oolong tea, which also are processed from traditional tea tree leaves.

Bukowski said L-theanine is broken down in the liver to ethylamine, a molecule that primes the response of an immune blood cell called the gamma-delta T cell.

“We know from other studies that these gamma-delta T cells in the blood are the first line of defense against many types of bacteria, viral, fungal and parasitic infections,” he said. “They even have some anti-tumor activity.”

The T cells prompt the secretion of interferon, a key part of the body’s chemical defense against infection, Bukowski said.

“We know from mouse studies that if you boost this part of the immune system it can protect against infection,” he said.

To further test the finding, the researchers had 11 volunteers drink five cups a day of tea, and 10 others drink coffee. Before the test began, they drew blood samples from all 21 test subjects.

After four weeks, they took more blood from the tea drinkers and then exposed that blood to the bacteria called E-coli. Bukowski said the immune cells in the specimens secreted five times more interferon than did blood cells from the same subjects before the weeks of tea drinking. Blood tests and bacteria challenges showed there was no change in the interferon levels of the coffee drinkers, he said.

Bukowski said it may be possible to further isolate and refine L-theanine from tea and use that as a drug to boost the infection defense of the body.

The health effects of tea have been extensively studied. It has been linked to lower heart disease and cancer risk through the action of flavonoids, a type of antioxidant. Other studies have linked tea to helping combat osteoporosis, the brittle bone disease, and to relieving some allergy symptoms.