The number of mushrooms on Earth is estimated at 140,000, of which maybe only 10 % are known. Meanwhile, ca.14, 000 species that we know today, about 50 % are considered to possess varying degrees of edibility, and about 700 species are medicinal mushrooms. Medicinal mushrooms such as Ganoderma lucidum (Reishi, Lingzhi), Lentinus edodes (Shiitake, Xiang gu), Inonotus obliquus (Chaga, Hei hua mo) and many others have been collected and used for hundreds of years in Korea, China, Japan, and eastern Russia. Those practices still form the basis of modern scientific studies of fungal medical activities, especially in the field of stomach, prostrate, and lung cancers. It is notable and remarkable how reliable the facts collected by traditional eastern medicine are in the study of medicinal mushrooms. Mushrooms of their high fiber content, sterols, proteins, microelements and a low calorific value, are almost ideal for diets designed to prevent cardiovascular diseases as first suggested by Traditional Chinese Medicine. Several mushroom species have been studied for anti-inflammatory and antioxidant activities and patents have been established for these usages. Fruit-bodies of Ganoderma lucidum and Lentinus edodes have long been a major factor in folk medicine for the treatment of chronic hepatitis. Polysaccharides belong to a structurally diverse class of macromolecules, in which monosaccharide residues join to each other by glycosidic linkages to form polymer. It is noteworthy that, in comparison with other biopolymers such as proteins and nucleic acids, polysaccharides offer the highest capacity for carrying biological information because they have the greatest potential for structural variability. Mushroom polysaccharides exert their antitumor action mostly via activation of the immune response of the host organism.

The human immune system has a remarkable ability to distinguish between the body’s own cells, recognized as “self” and foreign cells, as “nonself”, to mediate the immune responses. Mushroom β-glucans are a large group of macromolecules that are not naturally synthesized inside the human body, so these compounds are recognized as non-self-molecules, which activate the immunity. Mushroom polysaccharides or β-glucans are thought to provide their anti-tumor action primarily through the activation of the immune response of the host organism. In most cases mushroom polysaccharides do not directly affect tumor cells.
Certain β-glucans from medicinal mushrooms appear to activate cell-mediated and humoral immunity via activation of different immune cells, leading to elimination of tumor cells or pathogens. The activated macrophages (containing β-glucans) preferentially attack dead cells and intracellular pathogens. These macrophages also produce cytokines that prime natural killer cells and T lymphocytes, both of which are cytotoxic to tumor cells, via different mechanisms. Natural killer cells secrete chemical substances that destroy tumor cells by bursting cell membranes. Neutrophils effectively destroy targeted cells by cell mediated phagocytosis. In this review were described the hypothesized mechanism of action for fungal β-glucans against cancer cells.