Charles B. Stephensen, Ph.D., is a Research Leader at the USDA Western Human Nutrition Research Center (WHNRC) in Davis, CA and an Adjunct Professor of Nutrition at U.C. Davis. He has a longstanding interest in research on the effect of nutrition on immune function, with a particular focus on the effects of vitamin A, as well as vitamin D, on immune function. Recent work has focused on vitamin D and immune function in pregnancy, and on the health benefits of vitamin D supplementation during HIV infection. Ongoing work at the WHNRC is examining the associations of typical diet with the intestinal microbiota and the potential impact of the microbiota on systemic immune activation and inflammation.