Mushrooms

Mushrooms contain beta-glucans which are shown to stimulate the immune system. (along with oats, barely... etc)

http://www.pointinstitute.org/wp-content/uploads/2013/01/Point-Institute-Mushroom-Paper-1-25-13.pdf

http://en.wikipedia.org/wiki/Beta-glucan

http://www.mykosan.com/medicinal-mushroom-research.html

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3339609/#!po=71.2500

oldest man alive eats skunk and mushrooms

http://worldnews.nbcnews.com/_news/2013/08/18/20076751-quinoa-mushrooms-and-coca-bolivian-says-ancient-andean-diet-has-kept-him-alive-for-123-years?lite