This invention provides compositions and methods for stimulating the immune system. Such methods include administering an extract, purified peptide-linked glucan or active component thereof from Coriolus versicolor. The methods are particularly useful for prophylactic and therapeutic treatment of secondary immunodeficiency, wherein the immunodeficiency is the result of an infection, a malignant neoplastic disease, an autoimmune disease, a protein losing state, an immunosuppressive treatment, surgery or anesthesia.