Urticaria or hives can be idiopathic or may be a symptom of underlying problem in some individuals .
In perhaps as many as 30-40% of cases with chronic idiopathic urticaria, it is caused by an autoimmune reaction. Acute viral infection is another common cause of urticaria. Less common causes include friction, pressure, temperature extremes , exercise and sun light.
Angioedema is a related condition (also from non allergic causes) , though fluid leakage is from much deeper vessels.
The swelling appears around the mouth, eyes, throat, abdomen, or in other locations. Urticaria and angioedema sometimes occur together in some cases in response to an allergen.
Individual hives that last for more than 24 hours or leave a bruise as they heal are more likely to be due to urticarial vasculitis.