8 days ago in the er i was prescribed 10mg dexamethasone sodium phosphate through iv. would this one time dose affect my immune system? and how long does this typically stay in your system? im ready for it to be out of my system?

Not significantly: This intravenous steroid just mimics the natural stress response of the body (steroid production) but at a higher level. That single infusion is gone from your system now lasting at most a few days. Daily infusions at a therapeutic (high) dose over days & weeks would suppress the immune response for a significant period. Single infusions have only a mild & transient effect.