An allergen is any substance that causes an allergic reaction in someone's body. Common allergens include pollen from trees and flowers, certain foods like peanuts or shellfish, pet dander (tiny flakes of skin that animals shed), dust mites, and bee stings.

When your immune system encounters an allergen, it mistakenly treats it as dangerous, even though the substance is actually harmless to most people. This triggers a reaction that might cause sneezing, itchy eyes, hives, or, in serious cases, difficulty breathing. It's like your body's defense system has a faulty alarm that goes off for things that aren't really threats.

Different people react to different allergens. Your friend might sneeze around cats while you're perfectly fine, or you might break out in a rash from eating strawberries while your sister eats them without any problem. Scientists still don't fully understand why some people develop allergies while others don't, even when they're exposed to the same substances.

People with severe allergies often carry medicine like an EpiPen and learn to carefully avoid their specific allergens. Reading food labels, asking about ingredients at restaurants, and knowing what triggers a reaction can become an important part of staying safe and healthy.