ymav,Agreed with everything you've said, however, while the...

ymav,
Agreed with everything you've said, however, while the immune response is no guarantee that it will destroy cancer cells, it's not quite as prosaic as your example suggests.
Perhaps a better example using your analogy is that the 'strength' drug you ingest triggers measurable increase in protein synthesis.
It doesn't guarantee that will translate into muscle growth (or you will become stronger).
But it's an actual pharmacodynamic effect that's required for the desired benefit, rather than simply being able to detect the drug in your bloodstream.