Hypersensitive reaction?

2 y.o. Shih Tzu. We found a small area on her left hip that had an
irregular shaped roughness under the skin. The vet did a needle
biopsy and found 'round' cells and suggested the tumor-like subskin
growth be removed surgically. We did so. Pathology reported back
that the removed material was simply dead cells (necrotic tissue).

The vet suggested that the dog may have a hypersensitivity to the bite
of perhaps a tick (although no tick parts were found), spider, or
other insect. She doesn't seem to have any other allergies to
anything we've found.

My own google search comes up pretty empty on this type of condition.
Can anyone point me to some relevant URL's?

Thanks,
Derf