The outcome of a stress response is not a switch but a position on a continuum. At one end, the cell adapts: it recruits additional capacity, restores the set-point, and may end up more resistant to that stressor than it was before. In the middle, the cell is injured but the injury is reversible — structure and function are disturbed, yet removing the stressor allows full recovery. At the far end, injury becomes irreversible and the cell dies, typically by a regulated program rather than by passive collapse.
Three variables move a cell along this continuum. Intensity sets how large the initial mismatch is and therefore how much extra capacity must be recruited. Duration determines whether the recruited programs finish the job or are held open long enough to become damaging in their own right. Cell type sets the available reserve: a cell with abundant chaperones, high antioxidant capacity, and flexible metabolism tolerates a load that would overwhelm a cell with little of any of these.
Intensity and duration interact rather than acting independently. A very intense stressor can kill within minutes, before any adaptive program can be expressed. A mild stressor that would be fully corrected in an hour can become injurious if it persists for days, because sustained activation of protective programs carries its own costs. This is why the same pathway can look protective in one setting and harmful in another — the position on the continuum, not the pathway's identity, decides the outcome. Reversibility is the practical dividing line: as long as removing the stressor restores normal function, the cell is in the adaptive or reversibly injured zone; once removal no longer rescues it, the cell has crossed into irreversible injury.