A healthy immune system is dependent upon many different factors, some of them being nutritional. Antioxidants in general, zinc, copper, lysine and other nutrients all play an important role. Of all of these, Vitamin C is probably most important to a healthy immune system. Vitamin C is depleted under any type of stress, which is one reason why high stress levels can depress the immune system. There is a wide body of scientific evidence that indicates that Vitamin C and these other nutrients help nourish the immune system.