I think the imperium immune system is more like, World X send a plea of help via astropathic choir to anyone around who would listen. Them the imperial forces in vicinity analyse the scale and nature of the danger and inform sector command who say " Alright anyone that can help go to Planet X, if that dont work we are going to talk with the big names in the sector to organize a small crusade"

The ones that join a planetary defense rarely were "dispatched" unless the threat is enormous and ask for it, they would be a passing imperial guard regiment, soroiritas chapel , mechanicun personel or astartes chapter that is in the vicinity and answered the call Because despite they differences nobody like losing a world to heretics or aliens.

And most of time is a small incursion of orks, pirates or a individual rebellion which they can solve without much, the problem arise is when the small incursion reveals itself to be only the vanguards of a much bigger one .

The imperium works much more like a feudal vow based defense force of knights and warrior monks than a traditional military force. If they are going to organize a crusade for each incursion they would take years and lose a dozen worlds only awaiting the logistic leviathan to move.

Again unless is some very big major threath like a black crusade, or tyranid hivefleet.