Immunity is the ability of an organism to resist bacterial and viral infections. Immunoregulatory functions are believed to play a central role in maintaining a normal immune system so that an overactive response is suppressed and a weak response is upregulated. In other words, maintaining the balance is critical for the immune system. Since an overactive immune response can result in the development of an autoimmune disease and allergy, trying to actively regulate the immune system and maintain it at an ideal condition is worthy of consideration. As part of cell-mediated immunity, immune cells such as macrophages and NK cells play a central role in eliminating pathogens or pathogen-infected cells. On the other hand, humoral immunity is mediated by antibodies. Activated cell-mediated immunity is effective in defending the body against the initial infection, whereas its overresponse toward own cells can cause an autoimmune disease. In contrast, although the dominance of humoral immunity facilitates the production of antibodies against foreign substances, its overresponse can induce the production of antibodies against harmless substances and thus elevate the risk of an allergy. The normal immune system is maintained by achieving a balance between cell-mediated and humoral immunity. Cytokines, proteins that are produced by immune cells in response to stimulation, play an important role in cell signaling.