A doctor or nurse will put a small drop of liquid on your skin. This liquid usually has a tiny amount of an allergen in it. The allergen is the thing you might be allergic to, like pet dander, pollen, or dust mites. The doctor or nurse will then use a small tool to prick or scratch your skin with the allergen. If you're allergic to something, you'll usually develop a red, itchy bump on your skin within about 20 minutes.