What is the best supplement to take for an IGA Deficiency (immune syst

I have an IGA Deficiency that affects my upper respiratory system. What are some supplements I can take that would help build/protect me from infections?
I do take the pneumonia shots and the flu shot each year. I have been on LifeVantage ‘Protandim' for the past two years. It has been the only thing that keeps me from getting sick each school year, but it is so expensive! Any suggestions with results would be appreciated. Thank you!