Cholinergic urticaria is a subcategory of physical urticaria ( hives) that is a skin
rash brought on by a hypersensitive reaction to body
heat. Symptoms follow any stimulus to sweat such as exercise, heat from the sun, saunas, hot showers (reaction to water can also indicate water urticaria) or any stimulus that causes an increase in body temperature.
All urticarias are caused by an elevated histamine release by the body's mast cells.