There are three fairly common leaf galls on Celtis induced by feeding psyllid larvae

Unfortunately this doesn't seem to be a typical form of any of them! Blister Galls aren't stalked, Nipple Galls....well they aren't this shape, and we'd see Petiole Galls somewhere else on the plant. I'm sure someone can identify these galls. (hint hint) They're attractive in a curiously horrible sort of way.

If they are psyllid galls the literature suggests that trees don't suffer appreciably from extensive yearly infestations. Hard to believe. The insects, when they emerge as adults will obviously be numerous (every leaf on the tree is covered in galls) and apparently can be annoying in quantity. I guess that's the price we pay to see the galls!