Acute hives last less than six weeks and can be triggered by many factors. Foods such as nuts and shellfish usually cause fairly immediate symptoms. In addition to the hives, these patients usually have other symptoms of an immediate allergic reaction, such as throat closure, abdominal pain and difficulty breathing. Tylenol rarely does. In younger children, a viral illness such as a bad cold can trigger hives.
Chronic urticaria refers to hives that last more than six weeks.
The raised, red lesions and swelling are caused by specific cells in the skin called mast cells releasing histamine. Histamine causes itchy, runny noses and watery eyes in hay fever sufferers. Certain physical stimuli can trigger a reaction. In cold urticaria, a person exposed to low temperatures, for example after a plunge into a swimming pool, can break out with hives. Cholinergic urticaria refers to hives triggered by an increase in body temperature.