The decrease in the immune response can be correlated with cortisol, cortisol will be released by the body on a fight or flee situation, especially when it's triggered by adrenaline, that cortisol is going to break down muscle, fat, and carbs so that you get more available energy to actually fight or flee.

We weren’t built to be in a constant fight-or-flight mode, even though our current environment tends to force us to.

This is a huge problem nowadays, ppl who work stressful jobs are less likely to have their hormone system in check, making them fat, obese, diabetic, increased heart rate, increased blood pressure and, don't quote me on this one, even suicide might be related to unstable hormones.
Great post Suesa :)