A study comparing over 100,000 people with stress-related disorders - including Post Traumatic Stress Disorder (PTSD) - with their unaffected siblings has found that those with the disorder are much more likely to develop some forms of auto-immune disease.
One finding of preventative value was that people treated for PTSD (including prescribed SSRIs) were less likely than untreated people to develop the AI condition. So - if you can - get treatment for psychological trauma!
The study doesn't provide evidence of the mechanism involved, but one of the authors suggests it may be due to stress causing disruption of the hypothalamus-pituitary-adrenal (HPA) axis that regulates autonomic function. That may be expressed in lowered cortisol levels and increased inflammatory responses.
Makes a lot of sense to me!