A randomized dietary intervention study in healthy adults published 2015 has found that consuming Shiitake Mushroom(Lentinula edodes) daily improves various aspects of human immunity.
Shiitake mushroom is one of the most popular edible mushrooms in the world, second only to the button mushroom.
The cultivation of shiitake dates way back to 1100 A.D. during the rule of Sung dynasty which is replaced by more modern and efficient sawdust substrate log cultures lately. 
There is now a growing body of scientific evidence that has found various “medicinal” mushrooms such as Shiitake, Reishi, Chaga, Maitake, Lions Mane and Turkey Tail for example, may exert beneficial effects upon various aspects of the human immune system.
Not to mention possessing many other beneficial effects such as potent anti-viral activity to anti-inflammatory and immunomodulative properties.
Consuming Lentinula edodes (Shiitake) Mushrooms Daily Improves Human Immunity: A Randomized Dietary Intervention in Healthy Young Adults.
Eating L. edodes for 4 weeks resulted in increased ex vivo proliferation of γδ-T cell proliferation (60% more, p < 0.0001) and NK-T (2-fold more, p < 0.0001) cells.
Both cell types also demonstrated a greater ability to express activation receptors, suggesting that consuming mushrooms improved cell effector function.
The increase in sIgA implied improved gut immunity. The reduction in CRP suggested lower inflammation.
The pattern of cytokines secreted before and after mushroom consumption was significantly different; consumption resulted in increased interleukin (IL)-4, IL-10, tumor necrosis factor (TNF)-α, and IL-1α levels, a decreased macrophage inflammatory protein-1α/chemokine C-C ligand 3 (MIP-1α/CCL3) level, and no change to IL-6, IL-1β, MIP-1β, IL-17 and interferon (IFN)-γ levels.
The study concluded:
Regular L. edodes consumption resulted in improved immunity, as seen by improved cell proliferation and activation and increased sIgA production.
The changes observed in cytokine and serum CRP levels suggest that these improvements occurred under conditions that were less inflammatory than those that existed before consumption. 
 The Unexplored Anticaries Potential of Shiitake Mushroom
 Consuming Lentinula edodes (Shiitake) Mushrooms Daily Improves Human Immunity: A Randomized Dietary Intervention in Healthy Young Adults.
The information in this article has not been evaluated by the FDA and should not be used to diagnose, cure or treat any disease, implied or otherwise.