- Skin tests to examine the body s reaction to allergens-common environmental substances, such as cat dander or pollen-that cause an allergic reaction. The test is done in one of two ways: either the skin is lightly scratched and an allergen extract is placed over the just-broken skin, or a very fine needle is used to inject a small amount of allergen under the skin. In both methods, the site is monitored for swelling or hives in the next 48 hours.