Allergy is a common problem affecting millions of people around the world. Allergies occur when the body's immune system overreacts to certain substances, causing allergic reactions. The most common allergic reactions are sneezing, itching, and wheezing. In medical terms, the condition is referred to as being allergic to something. Some common synonyms for the phrase 'allergic to' include 'sensitive to,' 'intolerant of,' 'reactive to,' and 'hypersensitive to.' Allergy sufferers may experience a range of reactions from mild to severe, depending on the substance they are allergic to. Allergy testing and avoidance of allergens are the best ways to manage allergic reactions and prevent complications.