While conventional wisdom says an apple a day keeps the doctor away, evidence is mounting that a cup of tea may be a better way to go.

Researchers at Harvard University and the Brigham and Women's Hospital in Boston have discovered yet another way that drinking tea is good for you. A study published in the Proceedings of the National Academy of Sciences in April found that a substance in tea may help fight infection and strengthen the body's immune system.

In initial in vitro tests, researchers mixed gamma delta T cells, a main weapon for the immune system, with ethylamine, which is produced when the liver breaks down the amino acid L-theanine. L-theanine is found in high concentrations in black, green, oolong and pekoe teas. It's also found in bacteria, vegetables and wine.

The mixture resulted in a tenfold increase in gamma delta T cells, producing high levels of disease-fighting chemicals and a stronger immune response.

Subsequent tests compared volunteers who consumed approximately 20 ounces of tea to a group drinking coffee. After four weeks, blood samples from the tea drinkers showed five times more disease-fighting chemicals than samples from the coffee drinkers.