Shiitake mushrooms, in vivo studies, show an increase in immune response when taken as food.  

Enoki mushrooms too are considered immune stimulating a favorite of both Japanese and Chinese traditional medicine practitioners.

Commercially grown on cut oak trees.

(Other mushrooms)
Matsutake, an expensive Japanese favorite, known immune stimulating chemistry.

Maitake another Oriental mushroom is immune stimulating, anti-cancer.  Beta glucan a polysaccharide in the mushroom may account for the anti cancer effect in laboratory animals.  The anti-metastic activity has exhibited itself in a small number of human studies.

Five hundred milligram tablets of maitake lowered blood pressure in humans.  The mushroom also lower cholesterol in rats.

Enoki mushrooms now available (by request) in numerous supermarkets is one of the finest health protecting mushrooms.  Close up above of head and stalks:  heads about a 1/4 inch, stalks 1/4 inch wide to 5 inches in length.  Delicate.