Are you referring to his 'tattoos'? Maybe he was part of a tribe or something?
---------- Post added at 11:45 PM ---------- Previous post was at 11:36 PM ----------
I think Igneel also 'borrowed' some magic from Natsu, same with the other dragons. Grandiney says that they (dragons), hid in their dragon slayers to also prolong their own lives. Plus if you look at the dragon slayers who had dragons inside them, you'd notice that Cobra or Laxus were stronger than them. Laxus was defeated, and it took 2 dragon slayers to do it, heck Natsu would've been killed if it weren't for Gajeel. And Cobra, only reason Natsu won, as 'funny' as it was, the dude had really good hearing, and Natsu's roar made him 'deaf' not really, but you get the point (also before someone says "Jellal defeated Cobra..." he was in a prison, how the hell would've 'trained' if he couldn't use magic lol). Or at least that's how it looks to me, and really if you think about it, it kinda makes sense.
I also think that Acnologia would've still defeated Igneel, and Igneel was at the top.
I'll throw this anyway, regarding the dragonification 'antibodies', do you think there's a possibility that Natsu (since we saw that movie pic), can dragonize half of his body? Igneel mentions that's ALMOST impossible for them to turn into a 'Acnologia', but they (the dragon slayers), could possibly dragonize half of their bodies, what do you think about that?