Depressed immune system?

I am over 3 years out from RNY and it's only September and I am already sick with my first cold. I feel like ever since my surgery my immune system is severely compromised and I can't fight off anything. Every winter season from September through March I have recurring colds, sinus infections,  and bronchitis. I am constantly on antibiotics and even my friends have commented that it seems like this has been an issue since my surgery. 

My blood work always comes back normal and I even look at the report just to make sure. Does anyone else feel the same way and do you have any suggestions for boosting my immunity? At 3 years out my portions are pretty normal and I can eat quite a bit. Several weeks ago I made a conscious effort to begin eating more fruits and veggies, especially since I just had shoulder surgery and can't exercise the way I want. I don't want to face another winter of constant illness. Any insight out there is much appreciated.