Hives occurs when the body reacts to an allergen and releases histamine and other chemicals from under the surface of the skin. The histamine and chemicals cause inflammation and fluid to accumulate under the skin, causing wheals.
Examples of known Hives triggers
Medications, including some antibiotics and non-steroidal anti-inflammatory drugs (NSAIDs), such as aspirin and ACE inhibitors, used for high blood pressure.
Foods, such nuts, shellfish, food additives, eggs, strawberries, and wheat products
Infections, including influenza, the common cold, glandular fever, and hepatitis B
bacterial infections, including urinary tract infections and strep throat
Extreme temperatures or changes in temperature
High body temperature
Pet dander from dogs, cats, horses, and so on
Cockroaches and cockroach waste
Some plants, including nettles, poison ivy, and poison oak
Insect bites and stings
Chronic illness, such as thyroid disease or lupus
Water on the skin
In over half of all cases, people never find the exact cause.