Immunostimulant is a reasonable way to put it: it takes the brakes off one part of the immune system. (Similarly, we call caffeine a central nervous system stimulant and it works by blocking adenosine, which is one of the brakes on brain activity.)

And it's not always specific to tumours. My wife's thyroid got wiped out. The endocrinologist said it was like bombing a paint factory: first a massive spike in thyroid hormones, then a crash as no more were produced.

More info on the potential side effects of immune checkpoint inhibitors: https://www.cancer.gov/about-cancer/treatment/types/immunoth...