What Causes Hives?

Hives and angioedema can be divided into two categories based upon the duration of symptoms: acute and chronic. Acute hives are defined as lesions that come and go for a period of 6 weeks or less, and frequently the outbreak may last for just a few hours or days. Hives that last for a few hours to days are frequently caused by an allergy to a food or an insect sting, while hives that last between several days and a couple of weeks are typically seen with drug allergy or following common viral infections.

Chronic hives and angioedema are defined as lasting for longer than 6 weeks. These patients have hives alone in 40% of cases, hives and angioedema in 40% of cases, and angioedema alone in 20% of cases. In two-thirds of people with chronic hives, no specific cause can be identified (also called “idiopathic”). Approxi-mately 20–30% of patients with chronic hives are considered to have autoimmune hives; these patients have been shown to have antibodies that are directed against molecular targets in their own skin. As a general rule, these patients do not progress to develop other more serious autoimmune diseases, such as systemic lupus erythematosus or rheumatoid arthritis. The next most common cause of chronically recurring hives is a physical stimulus. These triggers include pressure applied to the skin, exposure to cold water or air, or increase in body temperature due to hot water or exercise.

The most frequently encountered of these stimuli is physical pressure, also called “dermographia.” The underlying basis for these physical causes of hives is not well understood. Other less common causes of chronic hives include systemic diseases such as hormonal disorders (e.g., thyroid disease), and chronic infections (e.g., intestinal parasites). Importantly, when systemic dis-eases cause hives or angioedema, there are usually other signs and symptoms that suggest the underlying problem in addition to the hives.

In addition to these diseases, there is an extremely rare disorder, called “hereditary angioedema,” in which the absence of an important blood protein (C1 esterase inhibitor protein) leads to recurrent acute bouts of deep skin swelling. These patients may have angioedema of the skin, throat, tongue, and/or gastrointestinal tract without any observable hives and the disorder usually becomes symptomatic by the fifth decade of life.