Erm...NO, Senora Bush..this whole country is NOT made up of immigrants.
Not that it matters but AN OVERWHELMING MAJORITY of us are NATIVE to this country and did NOT immigrate here.
I was perusing the thread and saw your comment. I cannot find the data at my fingertips, but I believe this is no longer true.(if you consider 1st and second generation)
If you use birth in the USA as a defining parameter (which Mrs. Bush was not), then it would be true.(barely)