As the weather starts getting colder, colds and flu symptoms will be coming out of the closet so now is the time to build up your immune system. White blood cells help defend your body against infectious disease. So by eating foods that increase the number of white blood cells such as garlic, shitake mushrooms, dark meat turkey and yogurts with live cultures you can help your immune system. Fruits and veggies do not increase the number of white blood cells your body produces but they make them stronger. Keep in mind that too much fat other than the kind found in fish oil will lower the strength of the immune system.