I was diagnosed 6 years ago with Major Depressive Illness. I also went through a whole range of antidepressents and became resistant to each one after a short period of time.

My Dr. prescribed a very tiny dose of Risperidone to in his words "coax the antidepressant to work". Of course this drug is normally prescribed to psycotic patients so the dose was really tiny, I looked it up at the time and if I'm remembering correctly it was a very small percentage of what they would give to someone who would take it as an anti-psychotic.

It worked like a charm! I was on the tiny dose plus my Effexor XR for about a year and then I just didn't need either anymore!

Keep at it, I don't think you are so much immune as you may just need that extra 'kick-start'.