Preventing allergic reactions depends on the type of allergy you have. General measures include the following: Stay away from known triggers. Do this even if you get treatment for your allergy symptoms. If, for instance, you're allergic to pollen, stay inside with windows and doors closed when pollen is high. 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.