Y'know, I never bought the whole "Spock is just a half-human" thing... this explains a lot...

Certain creatures are immune to certain things; my statement was perhaps imprecise. There is no such thing as INHERENTLY, COMPLETELY immune to ALL magic in HP.

I'm not sure what you mean about magic fading insofar as the Potterverse is concerned, and that "many secrets have been lost". In HP, magic is a dynamic force available to everyone. There is constant innovation and experimentation and discovery. Very rarely do you hear someone say, "That is an Olde Magike, lost to the ages of history." - unlike in LOTR, where everyone knows that magic is extremely rare and only usable if you're magically awesome. (Which, unfortunately, you are not. Sorry, readers at home out there in Middle Earth!)