Does bcg treatment for bladder cancer decrease your immune system?

See below : Intravesical BCG used to both treat and prevent recurrence of urothelial bladder cancer. Despite its widespread common usage, its specific mechanism of action poorly understood. BCG is an attenuated form of the bacteria that causes TB (much like a vaccine) that "stimulates" (not weakens) ones immune system in such a way to kill the tumor cells. Pts with weak immune systems have poorer responses.