•Foods (commonly eggs, shellfish, nuts, berries, dyes, or other additives)
•Drugs (any drug can touch off an allergic reaction, although allergies to penicillin, sulfa, and aspirin are especially common)
•Pollens and plants (nettles, poison
ivy, poison oak
, and so on)
•Insect bites or stings
•Exposure to heat, cold, or sunshine
Hives are a nuisance, but are not usually dangerous.