An allergic reaction triggers hives, and common triggers of an allergic reaction include pollen, animals, insect bites, medications, and foods, particularly nuts, shellfish, eggs, milk and citrus fruits, states the American Academy of Dermatology. Allergy shots and skin contact with certain things, such as latex, can also lead to hives.
Hives can be caused by exercise; sicknesses, such as thyroid problems, lupus and vasculitis; infections, such as colds and fungal or bacterial infections; and exposure to water, cold, heat or the sun, according to the American Academy of Dermatology. Hives also occur due to skin scratches, chemicals, skin pressure or stress. After being exposed to a trigger, individuals experience hives within several minutes or after two hours.
Signs of hives include swollen skin with a pink or red color, individual or group welts, and swelling that diminishes within 24 hours but appears at other areas of the skin, notes the American Academy of Dermatology. The skin condition causes itching, stinging or pain.
Hives that always occur at the same parts of the body are known as fixed hives, as they do not move to other spots, explains the American Academy of Dermatology. In people with fixed solar urticaria, the trigger of fixed hives is excessive exposure to sunlight, while those who experience fixed drug eruption suffer hives from taking certain medications.