Bugsy is a 9-year old English Bulldog, whose chief job is keeping the hearth rug from being sucked up the chimney. He's an indoor dog, an Only Pet, and lives with a late-middle-aged couple whose children have left home. In short, he's living high on the hog, spoiled rotten and getting plenty of attention.
Bugsy's "Mom" is petting him and rubbing his belly one day and notices a reddish, somewhat inflamed bump on his belly. It's about half an inch in diameter. She decides to keep an eye on it, thinking it may be an allergic reaction to something he encountered in 
the back yard, or perhaps a mosquito bite. She doesn't know if dogs get a reaction to mosquitoes but it seems to her they might.
The next day the lesion is somewhat smaller and less obvious, but in a few more days it becomes larger and more angry-looking and there's a hint of a second one forming nearby. She bundles Bugsy into the car and off he goes to the vet's office.
After hearing the history and noting the variable appearance of the lesions, the vet draws blood and sends it off to the lab; then takes an aspirate from the bump and makes a smear, which she stains with toluidine blue. After looking at this (left) she makes a preliminary diagnosis of a cutaneous mast cell tumor and advises Bugsy's owner to have it out, right now. She also recommends Bugsy for a number of tests such as abdominal ultrasound.
Points to ponder:
1. What connective tissue cell(s) is (are) involved in this lesion? What's the normal behavior of such cells?
2. What's causing the redness and irritation at the site of the tumor? Is the owner totally off base to think of this as some sort of an allergic reaction?
3. If this tumor had been undetected for another couple of months—if, say, Bugsy's doting owners weren't in the habit of giving him belly-rubs on demand—what systemic symptoms might have been clues to a problem, and how would they have presented?