When you're allergic to something, your body treats a normally harmless substance like a dangerous invader. Your immune system, which usually protects you from actual threats like viruses and bacteria, mistakenly attacks things like pollen, peanuts, or pet dander. This overreaction causes symptoms like sneezing, itching, hives, or a runny nose.

Some allergies are mild annoyances. A child allergic to cats might get watery eyes when visiting a friend who has one. Other allergies are severe and dangerous: someone with a peanut allergy might need an EpiPen and immediate medical help if they accidentally eat even a tiny amount.

Common allergens (things that cause allergic reactions) include certain foods, pollen, dust mites, bee stings, and medications. Scientists still don't fully understand why some people develop allergies while others don't, though allergies often run in families.

People also use allergic figuratively to mean a strong aversion to something. If your sister says she's “allergic to homework,” she means she really dislikes it, not that it makes her break out in hives.