Can you offer an example of when it would be required (or useful) to replace the bodies immune system with an alternative? Any foreign substance added to the b

Can you offer an example of when it would be required (or useful) to replace the bodies immune system with an alternative? Any foreign substance added to the body (even if it's fighting an infection) still needs to be cleaned out by the immune system. So adding viruses that attack bacteria isn't a zero sum game.