An immunomodulator is a substance that helps to regulate or modify the immune system's response. These agents can either enhance or suppress the immune response, depending on the needs of the body. They are often used in the treatment of various conditions, including autoimmune diseases, where the immune system mistakenly attacks healthy cells, and in organ transplantation to prevent rejection. Immunomodulators can be natural or synthetic and include a variety of compounds, such as cytokines, monoclonal antibodies, and immunosuppressants. By influencing the activity of immune cells, these substances play a crucial role in maintaining immune balance and promoting overall health.