###### Abstract  
Selenosugars are a group of sugar derivatives of great structural diversity (e.g., molar masses, selenium oxidation state, and selenium binding), obtained as a result of biosynthesis, chemical modification of natural compounds, or chemical synthesis. Seleno-monosaccharides and disaccharides are known to be non-toxic products of the natural metabolism of selenium compounds in mammals. In the case of the selenium-containing polysaccharides of natural origin, their formation is also postulated as a form of detoxification of excess selenium in microorganisms, mushroom, and plants. The valency of selenium in selenium-containing polysaccharides can be: 0 (encapsulated nano-selenium), IV (selenites of polysaccharides), or II (selenoglycosodes or selenium built into the sugar ring to replace oxygen). The great interest in Se-polysaccharides results from the expected synergy between selenium and polysaccharides. Several plant- and mushroom-derived polysaccharides are potent macromolecules with antitumor, immunomododulatory, antioxidant, and other biological properties. Selenium, a trace element of fundamental importance to human health, has been shown to possess several analogous functions. The mechanism by which selenium exerts anticancer and immunomododulatory activity differs from that of polysaccharide fractions, but a similar pharmacological effect suggests a possible synergy of these two agents. Various functions of Se-polysaccharides have been explored, including antitumor, immune-enhancement, antioxidant, antidiabetic, anti-inflammatory, hepatoprotective, and neuroprotective activities. Due to being non-toxic or much less toxic than inorganic selenium compounds, Se-polysaccharides are potential dietary supplements that could be used, e.g., in chemoprevention. selenium-containing polysaccharides; Se-polysaccharides; Se-glycolosides; selenates; Se-supplemented; organoselenium +
Footnote †: journal: Applied Sci. The modification of the polysaccharides' structure carried out by biotechnological or chemical methods may cause a significant change in their biological activity [6; 7; 8; 9]. For example, incorporation of selenium (Se) in the polysaccharide molecules is one of the currently explored methods of modifying the structure and activity of these compounds [10; 11; 12; 13]. Selenium is one of the trace elements that are of fundamental importance to human health, being part of the antioxidant enzymes that protect cells against the effects of free radicals [14; 15; 16; 17; 18]. It plays an important role in a number of biochemical functions in humans and animals, such as antioxidant, immune function, reproduction, and thyroid hormone metabolism. Selenium appears to be a key nutrient in cancer prevention and inhibition of HIV progression to AIDS [19; 20; 21; 22; 23; 24; 25]. The important health effect of selenium is related to its impact on the immune system. Numerous reports on the immunomodulatory and antiviral activity of preparations containing selenium show that supplementation with this micronutrient may be beneficial for both prevention and treatment of viral infections, including coronaviruses [26; 27]. The mechanism by which selenium exerts antioxidant, anticancer, and immunomodulating activity differs from that of polysaccharides, but a similar pharmacological effect suggests a possible synergy of these two agents. This has led to a growing research interest in Se-polysaccharides in recent years, particularly the methods of incorporating selenium into the carbohydrate structure and the structure-activity relationship of these compounds. The approach to the problem of the Se-polysaccharides presented in the current review is based precisely on the analysis of the structure of these compounds and related changes in biological activity.