High-fat diets may make certain immune system cells more sluggish and less functional, compromising their ability to protect you from illness. Fortunately, cutting back on fat can kick your immune function up a few notches. Researchers at Tufts University compared the effects of a typical Western diet (containing 38 percent fat) with a cholesterol-lowering diet (28 percent fat) on immune function. The findings: the lower-fat diet enhanced the function of T-cells, a type of white blood cell that helps ward off infections. Your to-do: Curb fat intake, particularly saturated and trans fats (polyunsaturated and monounsaturated fats are healthy). For starters, try the Dietary Approaches to Stop Hypertension (DASH) diet or a Mediterranean diet, advises Dr. Schachter.