Actually, immunotherapy is the new old big thing in cancer research. As I recall it was first hot in the late 70s/early 80s after the invention of hybridomas, but failed to produce dramatic early successes.

I share the parent's skepticism. Fighting cancer is fighting basic biology population dynamics: the population that survives the assault is by definition harder to kill. It's made even harder by the very nature of cancer cells that freed from following the rules: they can relocate at will while simultaneously ignoring the body's zoning board (metastasis), steal nutrients (angiogenesis), dump waste in their backyard (poor lymphatic recruitment) and create an area that is inhospitable to other cells while simultaneously making for difficult drug delivery, ignore court orders to stop what they're doing (evade apoptosis), and replicate faster than other cells in the body (both by producing their own growth signals and ignoring anti-growth signals, including senescence). Also, the cancer cells have a head start. The deck is stacked against "curing" cancer by chemotherapeutics or immunotherapy.