Broccoli

Broccoli- a chemical in broccoli helped stimulate the immune systems of mice. Plus, it's full of nutrients that protect your body from damage.  It has vitamins A, vitamin C, and glutathione.  Add some low-fat cheese to round out a side dish with immune-enhancing B vitamins and vitamin D.

for more info- see-
http://www.webmd.com/cold-and-flu/slideshow-immune-foods?ecd=wnl_wmh_050911