Final answer:
A wheal is caused by the release of histamine and granule mediators in the wheal and flare response during skin testing.
Step-by-step explanation:
In skin testing, a wheal is caused by the release of histamine and granule mediators. It appears as a soft, pale swelling at the site surrounded by a red zone, known as wheal and flare response. The soft center is due to fluid leaking from the blood vessels and the redness is caused by the increased blood flow to the area due to the dilation of local blood vessels.