While food is not medicine, it can play a role in helping your immune system function optimally. At the very core, your immune system needs adequate energy and nutrients to respond to potential pathogens. During cold and flu season, you can focus on getting a balanced mix of protein and vitamins that help build up the body’s defenses. Specific vitamins to include in your diet are Vitamins A, C and E. Vitamin A helps nourish your skin, which acts as your body’s first line of resistance. Sources of Vitamin A include fish, cheese and eggs. Vitamin C helps your body form antigens and can be found in products that are typically acidic, such as citrus and tomatoes. Vitamin E acts as an antioxidant, which helps to slow potential body damage. Vitamin E can be found in various nuts and seeds.