Accumulating experimental and clinical evidence suggests that the immune response to cancer is not exclusively anti-tumor. In fact, several pro-tumor effects of the immune system have been identified, such as production of growth factors, establishment of angiogenesis, inhibition of immune response, initiation of cell movement and metastasis, and establishment of metastatic niches. Based on experimental data, we develop a mathematical model for the immune-mediated theory of metastasis, which includes anti- and pro-tumor effects of the immune system. The immune-mediated theory of metastasis can explain dormancy of metastasis and metastatic blow-up after resection of the primary tumor. It can explain increased metastasis at sites of injury, and the relatively poor performance of Immunotherapies, due to pro-tumor effects of the immune system. Our results suggest that further work is warranted to fully elucidate and control the pro-tumor effects of the immune system in metastatic cancer. (with Adam Rhodes).