Immunity is the state of resistance or insusceptibility exhibited by the host towards injury caused by micro-organisms and their products. Defense against infectious disease is one of the functions. Immunity, in general, is concerned with the reaction of the body against any foreign antigen. Although immunity is beneficial to the host, sometimes may be harmful also leading to tissue damage and disease (hypersensitivity and autoimmune diseases). Immunity needs to be modulated by stimulating the immune response to increase resistance to infection and by suppressing the immune response to treat autoimmune diseases, to prevent rejection of graft and hypersensitivity reactions. Immunity can be modulated by using immunomodulators- the agents that have the ability to boost or suppress the host defense response. Immunomodulating agents are synthetic chemotherapeutic agents or biological substances. Based on their ability to stimulate, suppress, or modulate the adaptive or innate immune system, they have been classified into three types.