"Fallen Angels" = aliens
"Nephilim"= descendants of 1st generation alien/human pair
Honestly, do you really believe there is any "pure" human blood left today? If you do, you may wish to rethink that.
"pure human" as in the blood of the original inhabitants of Earth. The bottom line is that at this point, there has been so much interbreeding between the "humans" and "aliens" that we are all hybrids to some degree or another. The "Bloodlines" of the elite have been manipulated to have a higher percentage of "alien" DNA, while the average person has less. We're all related, our respective DNA is just slightly skewed one way or the other.