Probiotics which are found in yogurt are good bacteria which pass through the intestinal tract. They aid in fighting off bad bacteria, which helps to boost immunity. Adding yogurt to the diet boosts the production of white blood cells which are part of our arsenal against invading germs. Kefir, a Middle Eastern food contains different bacteria that can live in the intestinal tract along with good yeast that fights off bad strains of yeast. Fruits and vegetables all are rich in vitamins and antioxidants and are healthy foods to help the immune system function optimally. Turmeric has cold-fighting properties and is found in foods made with curry. Garlic contains sulfur and has anti-viral, bacterial, fungal and parasitic properties. It is also contains antioxidants. Oregano, a spice used in Italian foods prevents infections. Bell peppers contain vitamin C which fights infections and boosts immunity. Green tea is known to have antioxidant and anti--cancer properties. Ginger helps increase perspiration to remove toxins from the body. Yellow vegetables like those in the squash family contain Vitamin A another immune system booster. Blueberries have antioxidants and are loaded with vitamins A and C. Greens like collard greens and kale are excellent sources of Vitamin A.
Vitamin C mostly tops the list when it comes to boosting immunity. You can find that in most citric fruits or in Vitamin C chews. Vitamin E is a vey good antioxidant and immunity booster as well. You can get this through you average grains,seeds and vegetable oils. And of course there is a whole lot like your Zinc, Carotenoids and all that.
Basically fruits, vegetables,dairy products, green tea (VRY HIGHLY RECOMMENDED), sweet potatos, spinach etc are all very good for boosting the immunity system.
Hope that was useful! :) Take care! :D