Abstract: In some embodiments, the present disclosure provides methods for stimulating a subject's immune system. Particular methods provide a tumor specific immune response by injecting a tumor with a magnetorheological fluid and applying a magnetic field to the tumor. The immune response can be used to treat disseminated tumors or metastatic cells. Further methods induce a general immune response, such as by treating a section of tissue, such as healthy tissue, with a magnetorheological fluid and applying a magnetic field to the treated tissue. The immune response can be used to treat illness or disease or to enhance the effect of a therapeutic agent. In particular examples, the disclosed methods are used to enhance the effect of immuno stimulants. Also provided are methods for causing necrosis of a cell mass, such as a tumor, by injecting a magnetorheological fluid into a section of tissue and applying a magnetic field to the tissue.