Do you put up a kettle at the first sign of a sore throat? You're on the right track, but drinking tea all winter long may help prevent you from getting that cold in the first place. L-theanine, a compound in black, green, oolong, and pekoe tea, strengthens the body's immune response when fighting off infection, according to researchers from Brigham and Women's Hospital in Boston. Bonus: Other studies suggest that the L-theanine in tea may help promote alertness without making you feel jittery.