The skin is pricked and exposed to tiny bits of common allergens found in the air. These include dust mites, mold, pollen, and cat and dog dander. If you're allergic to any of these, you'll likely get a raised bump where your skin was pricked. If you're not allergic, your skin won't have changes. Blood test. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.