The sudden spike in histamine levels can cause an outbreak of
hives in the upper layers of the skin.
Hives are often caused by a hypersensitivity to:
•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)
•Animal fur
•Insect bites or stings
•Exposure to heat, cold, or sunshine.