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This massive mushroom has been used in traditional Chinese medicine for centuries to boost immune system. With its high levels of fiber, amino acids, potassium, calcium, and magnesium -- as well as niacin and other B vitamins, it's no wonder that the 'shroom is relied on in traditional medicine. But Western medicine is also interested in the immune-boosting properties of this mushroom, in themaitake family: a 2009 study found that taking a maitake extract actually improved the immune system of breast cancer patients who were undergoing chemotherapy.