Ok... Vitamin D regulates the immune system. Since your D3 level is in the tank, then it makes since that the IgM would be low. That is to someone who is a total lay person and just did a quick google on IgM and read up on Wikipedia *grin*!
One thing though... my level was 12 not too long ago ('bout 3-4 year) and I'd been taking 1,000iu and it did nothing. I started taking 20,000iu daily after hearing from some other folks who had really low levels and what their doctors recommended to get it up. The general consensus was 20,000iu was needed to get it up. For me, 20,000iu daily got my levels up to 54 or 56 in 'bout 3 months. You want your levels in the 70's at minimum so you begin to store reserves, which is the latest word on D3 from vitamindcouncil.org