Mushrooms have been used for thousands of years to enhance immunity.  Mushrooms are high in beta glucans, which help protect us from infection. Beta glucans have been shown to prevent cancer progression and have synergistic anti-tumor effects when combined with monoclonal antibodies and certain chemotherapies.
 
References:
Immune Modulation From Five Major Mushrooms: Application to Integrative 
 
Effect of Beta Glucan on White Blood Cell Counts and Serum
Levels of IL-4 and IL-12 in Women with Breast Cancer Undergoing Chemotherapy: A Randomized Double-Blind Placebo-Controlled Clinical Trial
 
Orally Administered Beta-glucans Enhance Anti-Tumor Effects of Monoclonal