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. Patients will develop hives with exercise and hot showers. Four percent of the population has dermographism.
Hives occur when the skin is stroked with a firm object.
The pressure on the mast cells causes them to release histamine. Most of the patients I see in my practice do not fit into any of the categories mentioned. Their hives are not triggered by anything specific such as food, medications or heat. Their hives occur at random times, often in their sleep, without any obvious reason. These patients are usually very perplexed.