Natural exogenous immunostimulators are preparations isolated from bacteria, plants or fungi that activate immunological system. Present research is aimed at understanding the exact mechanism of their action and improving their efficacy in parallel with reducing minor side effects. It appears that in the majority therapeutical effects of the bacterial and fungal preparations result from the activation of Toll- like receptors (TLR). However, biologically active substances isolated from plants can activate cells through non-specific interaction with their surface.