Actually, the part about the EMTs believing she's rejecting a transplant is a serious case of Artistic Licence Medicine. Anyone with even basic medical education should be able to tell that these horns are not a "graft being rejected". Even artificial grafts take a while to heal, leave scars and give specific symptoms when rejected, and it goes double for transplants (especially xenogenic, that is, from another species). Now, any grafts inside the body are tricky, since you need advanced equipment to even tell they're there, but we're talking about horns here.
To simplify: unless Chloe's horns look like they're about to pop off her head (and we just don't see it due to art style limitations), a medical professional might assume that they're grafts, but won't be likely to blame her condition on them getting rejected. The same applies to her wings.
And unless non-supernatural body modification in the 'verse is on a much higher technical level than in our world, most people would not assume that these are real horns. They would be more likely to think it's an ingenious piece of characterization, such as
this ("damn cosplayers fainting in the street in their costumes"). Or, with appropriate knowlege about body mods, they could assume her horns are mounted on transdermal anchors (something like
this - page SFW, just don't explore the rest of the website), only wondering how the heck these anchors are still hanging in there with such a huge weight applied to them.
(Yeah, I know a transdermal anchor is basically a small graft - but still not a "horns grafted to skull" kind).
Of course, it's a comic strip. One with supernatural creatures. But the EMTs' assumption still feels out of place to me.
EDIT: There is one possibility where a graft could disagree with the host's organisms withot the telltale visual symptoms of rejection. It's called a "graft vs host disease", in which elements of the donor's immune system contained in the graft attack the host's organism. Obviously, this only works when the graft contains such elements (blood, bone marrow, etc.). And unless I'm mistaken, it requires the graft to be freshly implanted - which would, again, be obvious to the naked eye (recently wounded skin around the horns, swelling, stitches or vestiges of tissue glue...).
Sorry, I'm just one of those guys.
