What is the term for substances that suppress the body's immune response to antigens?

The term that describes substances which suppress the body's immune response to antigens is "immunosuppressive." Immunosuppressive agents intentionally reduce the immune system's activity to mitigate its reaction against foreign substances or to prevent it from attacking the body’s own tissues in conditions such as autoimmune diseases or during organ transplantation. By decreasing the immune response, these agents help in managing conditions where the immune system is overactive or misdirected.

Other terms provided in the choices refer to different functions related to the immune system. Immunostimulants enhance the immune response, promoting activity against infections or diseases. Immunomodulators can both enhance and suppress immune responses, depending on the context and specific mechanisms of action. Immunotherapies involve strategies that harness or modify the immune system to fight diseases, particularly cancer, but they might not necessarily suppress the immune response. Thus, the specific role of immunosuppressive agents in dampening immune reactions is what makes this term the most accurate in describing substances that reduce immune response.