BCG (Bacillus Calmette–Guérin, still a common base for TB vaccines today in many countries) has been a standard treatment for bladder cancer (specifically: non-muscle invasive bladder cancer) since the late 70s. As it was explained to me in medical school in the early 90s, before molecular biology was widely understood by physicians (at least not to my standards -- I was a molecular biologist before med school), it was a general stimulant of local immune response, but I always suspected it was something more specific.
The idea of a specific immunological cross-reactions has been well known in medicine for maybe 80 years. "Rheumatic fever" caused by Group A ß-hemolytic streptococci often triggers heart/valve damage because antibodies produced in response to a bacterial protein often cross-react with a structural heart protein. In this example (once called "rheumatic" heart disease or valve damage), the effects are negative. I always thought deliberately targeted cross-reactions were an obvious path for treatment investigation, and was frustrated that it never seemed to be very actively pursued.
In truth, it probably has been, many times. Some positive studies were likely published; others were equivocal or lack sufficient (statistical) power. Some failed.
As a space enthusiast, I always say "space is hard". Biology is harder. Even I forget that, mostly because a molecular biologist's first reaction is to try to think of easy ways to explore/prove their latest idea (trying hard to ignore the fact that their "quick elegant" experiment will likely take years to bear compelling evidence, due to complications) -- and a physician? Well, even we forget the truth/depth of the aphorism "it's an art as much as a science".
I'm hoping we'll be seeing a LOT more results along this line of inquiry in the coming decade, because I'm hoping we're finally ready to really explore it. We may not be, yet. Molecular simulations may not yet be at sufficient reliability, and the combinatorial math may yield too many permutations for empirical trials