Shitake, maitake, enokidake (enoki), cremini, portobello, oyster, and thistle oyster mushrooms all contain substances which stimulate and strengthen the immune system. In Japan, mushrooms are used as an immune-boosting compliment to chemotherapy. (Anti-Cancer, David Servan-Schreiber, page 123)
What about white button mushrooms?
White button mushrooms also have a significant anti-cancer effect.
For information about ALL the different medicinal properties of many different kinds of mushrooms, check out: Wikipedia.