[utensil4778]

Mechanically, all hemoglobin does is take up an oxygen molecule when in an oxygen rich environment, and releases it in a CO2 rich environment. CO2 just dissolves into the bulk fluid of the blood and diffuses out in the lungs, it is not carried by hemoglobin. However, hemoglobin can bind to CO carbon monoxide, and it can't let go of it. The hemoglobin molecule is effectively dead and useless.

If you had a molecule which functions in a similar way, it'd probably work in blood. The real trick is getting it to release the oxygen in the presence of CO2.

Immune response is a different matter that I don't know enough about to weigh in on. I'd assume there's ways around it. Possibly immunosuppressor drugs for the (hopefully) short time you're on the synthetic blood.