My dog is very allergic to stings, and seems to get stung once a year at least. I keep benadryl on hand at all times.
The first time she had a reaction, her neck swelled up, looked like she had the mumps. We rushed her to the vet, he gave her a cortizone shot and suggested benadryl.
This past Monday, she got stung evidently near her eye when she went out first thing in the morning. After coming in and sleeping for awhile, she woke at 9 or so with a totally swollen eye. We started her on the benadryl, but still took her in to the vet, since it was her eye and because I am paranoid. She was given another cortizone shot and after almost a full day, she is back to herself.
I guess some dogs are more allergic than others, just like humans (I have to go to the doctor everytime I get a sting from a wasp, bee, hornet, etc.).