•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
oak, and so on)
•Insect bites or stings
•Exposure to heat, cold, or sunshine.
For people with chronic hives, the simpler treatments may not keep outbreaks under control.