>if stress weakens the immune system, why is it that autoimmune disorders like psoriasis are exacerbated by stress?



very complex topic, and a lot is still not known, although understanding & research of autoimmune conditions has picked up in the last decade.



as you say, stress - chronic, harmful stress that for (multiple) whatever reasons is poorly adapted to - raises inflammation but also leads to higher rates of illness. this chronic stress as opposed to normal/controlled/healthy stresses, such as the correct amount of exercise.



unfortunately in autoimmune conditions it's not necessarily a question of "too much immune system activity" so much as an imbalance or "too much in certain systems" (e.g. tumor necrosis factor alpha, or as another example some research has talked about a ratio between IL-1 and IL-2) and maybe "not enough in others".



One mechanism I can throw out: the taxing of endogenous protective systems, such as glutathione, associated with chronic elevated stress markers. This definitely happens, and not only is low glutathione heavily associated with any autoimmune condition, but also the agent is a general detoxifier, and for example protects the liver, and if the liver becomes worse at removing toxins then plenty of problems will worsen ... etc.



endless loops within loops! almost as bad as neurochemistry when you look a little closely