Many people reach for the orange juice when it's cold and flu season, hoping that abundant vitamin C will keep them from coming down with something, even though there isn't much evidence that this works. But a new study suggests your immune system still can get a boost from fruit: in particular, red grapes and blueberries.

Researchers at Oregon State University looked at how 446 different chemical compounds impact the CAMP gene, which triggers the production of a jack-of-all-trades immune system protein that kills bacteria, promotes healing, and helps reduce inflammation. Of the compounds they tested, resveratrol (found in the skins of red grapes) and pterostilbene (from blueberries) boosted the gene's expression the most – and did so even more when combined with vitamin D, which is vital for a properly functioning immune system.

The bottom line: Eat more of them, especially blueberries, which are lower in sugar. "These are compounds you can easily get in your diet," says Adrian Gombart, a biochemist who led the study, "from fruit, juices, and red wine, of course."