Reaction after allergens removed?

DS went for more allergy testing today since it was a year since his previous tests. The nurse did the patch test, waited the 20 minutes and came back to read the results. She wiped his back with the alcohol and started measuring the reactions. After she was finished his back literally erupted with angry, red bumps. She seemed surprised that he was having a reaction after she wiped his back. I questioned if maybe when she was wiping him if maybe she rubbed one of his allergens all over his back? She put the antihistamine cream on his back and the bumps and redness went away, but it just seemed pretty strange. Anyone else had a similar reaction?
On a side note- he still has his milk, banans and apple allergies, but the oats, strawberry and pineapple are gone. He did however pick up an allergy to green beans. Who on earth is allergic to green beans?! Guess it just shows that people can be allergic to just about anything and everything.