After being poisoned by co2, i have had sinus infection, bronchitis 2x within 2mos...how do i build my immune system back up quickly?

Immune system health: Eating a balanced diet, getting adequate rest, and limiting physical and emotional stress are important in maintaining a healthy immune system. While there is no shortcut, adopting a healthy lifestyle will help recovery of immune system function.

Carbon monoxide: I think you mean CO (carbon monoxide), not CO2 which is a normal gas we exhale. How were you treated? Did you have hyperbaric O2? this can cause mucosal barotrauma in sinus and lungs? What you are experiencing is not due to immune deficiency, it is mucosal / cilliary injury. There is not much you can do except time. Avoid being around tobacco or other smoke. Keep air warm and moist. Inhaler PRN.