Researchers found that both fruits contain compounds called stilbenoids, which work with vitamin D to increase expression of the human cathelicidin antimicrobial peptide (CAMP) gene, which is involved in immune function.
The stilbenoid compounds included resveratrol in red grapes and pterostilbene in blueberries.
"Out of a study of hundreds of compounds, just these two popped right out," Adrian Gombart, a principal investigator at the Linus Pauling Institute at Oregon State University, said in a university news release.
"Their synergy with vitamin D to increase CAMP gene expression was significant and intriguing," said Gombart, an associate professor in the university's college of science. "It's a pretty interesting interaction."
Gombart and colleagues noted, however, that these findings were made in laboratory cell cultures and do not prove that eating blueberries and red grapes would boost a person's immune function.
The study was published Sept. 17 in the journal Molecular Nutrition and Food Research.
The CAMP gene has been shown to play a key role in the innate immune system -- the body's first line of defense that gives it the ability to fight bacterial infection. The response is especially crucial as many antibiotics become less effective.
Previous research has found a strong association between adequate vitamin D levels and the function of the CAMP gene. This new study suggests that certain other compounds may play a role as well.
