I've read that it's very important to have good Vitamin D levels. It would be nice to find out what your levels are the next time you go in for a physical. You can supplement with Vitamin D3 on a regular basis just to be sure because that does boost the immune system.
I also workout too much. I think you can tell if you just feel drained and exhausted.
The best way to keep the immune system strong is to eat a healthy diet consisting of whole foods, plenty of raw dark/leafy green veggies, lots of color in the diet (not just green but blue, red, orange), take a good probiotic, consume enough protein, and drink lots of water and things like white tea (which is supposed to be better than green tea).
I hope that helps a little.