Common causes of histamine release, which leads to allergic hives, include allergic reactions, consumption of certain foods, exposure to sunlight, medications and insect stings. Allergic hives occur when blood plasma oozes out of the skin's tiny blood vessels when the cells along these blood vessels release histamine, explains WebMD.
Citrus fruits, shellfish, peanuts, eggs and milk are foods that commonly trigger an allergic reaction that causes hives, states the American Academy of Dermatology. Allergy shots, pollen or skin contact with an item that causes allergy are also possible causes of hives. Some people experience hives due to bacterial or fungal infections, exercise, stress, skin pressure, or conditions such as lupus and thyroid disease.
Hives manifest minutes or hours after exposure to the trigger, notes the American Academy of Dermatology. People with hives usually have pink or red swollen bumps on the skin, individual or group welts, or swollen skin that diminishes at a particular spot within 24 hours and then appears at another area.
Hives are often itchy and sometimes painful, according to the American Academy of Dermatology. People with fixed hives have bumps or welts that stay on the same spots, and they tend to experience hives with exposure to certain triggers. Fixed drug eruption occurs when a person takes a medication that triggers an allergic reaction, while fixed solar urticaria results from exposure to excessive sunlight.