what can I do to strenthen my immune system while pregnant?

So I'm 26 weeks preggers..and have had a head cold / sinus infection on and off for about 3 months!! HELP! I have done the saline rinses, nasal washes, been on antibiotics ( 3x's) have had a double ear infection from it as well as had it drain into the root of my tooth causing a root canal to go bad....At this point..I just want yo get rid of it! But with all the limitation on you during pregnancy I'm not sure what I can do that is safe to strengthen my immune system to fight this off.. Does anyone know what vitamins other then my prenatals I can take? Dr has me on flonase, and I would really like to get off it as soon as I can...thanks for the help :)