Glossary On Causes

The primary mechanism that leads to development of an urticarial rash is the release of histamine (a chemical) by the human immune system into the blood. In response to histamine, blood plasma leaks out of small blood vessels in the skin resulting in swelling.

A large number of things can trigger the onset of Urticaria and some of them have been discussed here:

Allergens: A number of food allergens, environmental allergens, etc. can trigger Urticaria; however reactions may vary in different individuals

Foods: Some of the common culprits are fish, crabs, oysters, shell fish, prawns, milk, cheese, eggs, nuts, chocolates, citrus fruits, spices, yeast, food additives and preservatives like tartazine, berries, wheat, tomatoes, pineapple, strawberries, alcohol, etc.
Environmental: Pollen, dander, fungi, house-dust are few of the common reasons

Drugs: A number of drugs can trigger an Urticaria rash and these include morphine, codeine, aspirin, other nonsteroidal anti-inflammatory drugs (NSAIDs, such as ibuprofen), certain antibiotics, vaccinations, contraceptive pills, etc.
Stings of insects such as bee stings commonly cause acute urticarial rashes
Infections: Parasitic worm infestations such as round worms, tapeworms, etc; bacterial infections, etc can also cause Urticaria
Cold Urticaria is one variant that results from exposure to extremely cold climates
Solar Urticaria - results from exposure to sun
Pressure on the skin such as that caused by tight clothes may lead to pressure Urticaria
Physical exertion, exercise, hot shower, perspiration may trigger Cholinergic Urticaria
Direct physical stimulation of the skin by vibrations, firmly stroking or scratching the skin, etc.
Abnormal functioning of the immune system (auto-immunity) - has been considered to be a trigger for chronic Urticaria. Other auto-immune systemic illness such as thyroid disorders, systemic lupus erythematosus, etc. may be linked to Urticaria
Idiopathic - In about 80% of chronic Urticaria cases, the cause is unknown