http://en.wikipedia.org/wiki/Polysaccharide-K
The common English name of this mushroom is turkey tail. Supplements can be purchased at many health food stores.I don't know where fresh turkey tail mushrooms can be purchased.
Reishi and maitake are also believed to help the immune system during cancer treatment. Reading that it helps Herceptin is a plus.
Part of the reason I never needed shots for my white blood counts during treatment is because I took supplements containing turkey tail, maitake,reishi and shitake along with echinacea and astragalus. I cook with fresh, dried or frozen mushrooms whenever I can.