The proper functioning of human immune system is essential for organism survival against infectious, toxic and oncogenic agents. The concept of immunomodulation was proposed by Edward Jenner, while working on polio vaccine in 1796. A brawny, fine-functioning immune system is the keystone of excellent health. Immune replies are the consequence of an effectual interaction among innate (natural and non-specific) and acquired (adaptive and specific) components of the immune system. Inequity or failure of the immune systems is connected with a variety of chronic illness counting allergies, autoimmune diseases, cancers and furthers. Immunomodulators are natural or synthetic materials that regulate the immune system and induce innate and adaptive defense mechanisms. These substances are classified into two types, immunostimulants and immunosuppressants. Immunostimulants can enhance body's resistance against various infections through increasing the basal levels of immune response. These agents could increase the oxidative activity of neutrophils, augment engulfment activity of phagocytic cells and stimulate cytotoxic cells as necessary defense mechanisms. The researchers classified the immunostimulants using their origin and mode of action such as bacterial products, complex carbohydrates, vaccines (antigens and adjuvants), cytokines, immunoenhancing drugs, nutritional factors, animal extracts, and plant extracts. The link between immune system with diet, exercise, sleep, stress, microbial exposure, alcohol, water, hygiene are found to influence the immune response to a greater extent. Immunostimulants and some immunity enhancing nutrients improve the functioning of the immune system. In addition, some preventive measures such as healthy diet or proper nutrition, moderate exercise, sound sleep, drinking adequate water, subclinical exposure to microbes and managing stress, good hygiene, are altogether capable of boosting the immune system. In this review, the concepts, types and functions of immunostimulants will be described as a therapeutic approach against different diseases. Keywords: Immunomodulators, Immunostimulants, Adjuvant, Mechanism, Immune system.