Garlic
Garlic contains chemical compounds that are antibacterial, antifungal, antiviral, and even anticarcinogenic.  Studies have also shown that it can help lower cholesterol, blood pressure, and reduce the risk of blood clots and stroke.  Fresh garlic is best, as these beneficial nutrients are lost within one hour of smashing the cloves.  Add fresh garlic to your daily diet instead of supplements for this reason.

Mushrooms
Mushrooms are loaded with nutrients and minerals like vitamins B, C, and D, calcium, protein, and fiber.  The nutrients found especially in the Reishi, Shiitake and Maitake varieties are well known for their ability to activate and enhance the immune system.

Berries
Raspberries and blueberries in specific are extremely high in antioxidants that help boost your immune function by fighting off free radicals that can contribute to a number of health concerns.  Blueberries in particular can also keep bacteria from bonding to cells in the urinary tract, and therefore help prevent infection.

Broccoli
Broccoli is full of immune boosting nutrients including vitamin C, fiber, and even beta carotene which is good for the eyes as well as the immune system.  It also contains chemical compounds that have been shown to stop growth of cancer cells.