Why can Monoclonal Antibodies be Used to Treat Cancer? Why can't we use the Immune System?

-Cancer Cells have Antigens, sure, but its Antigens aren't Found on Normal Body Cells - this is a Tumour Markers -And, Cancer Cells are made by the Body so therefore aren't 'Foreign' for the Body meaning the Immune System will do Nothing -However, Monoclonal Antibodies can be Made to Bind onto the Tumour Markers, Effectively Treating Cancer.