•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
Treatment is with anti-histamines like Cetrizine/Loratidine etc. You can also apply Calamine lotion on the affected areas.
The reason for the reaction is genetically determined.