Actually "deranged" is a fairly common way of describing medical abnormalities not just the immune system.
https://khn.org/news/article/covid-autoimmune-virus-rogue-antibodies-cytokine-storm-severe-disease/
It sounds better than the immune system becomes "unhinged" and other terminology.

My personal favorite was that things were "running amok" as in violently raging, wild, or uncontrolled manner.