Watermelon

Watermelon
Hydrating and refreshing, ripe watermelon also has plenty of a powerful antioxidant, glutathione. Known to help strengthen the immune system so it can fight infection, glutathione is found in the red pulpy flesh near the rind.

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