Hives

Hives are raised, often itchy, red bumps (welts) on the surface of the skin. They are usually an allergic reaction to food or medicine. They can also appear without cause.

Causes

When you have an allergic reaction to a substance, your body releases histamine and other chemicals into the blood. This causes itching, swelling, and other symptoms. Hives are a common reaction. Persons with other allergies, such as hay fever, often get hives.

When swelling or welts occur around the face, especially the lips and eyes, it is called angioedema. Swelling can also occur around your hands, feet, and throat.

Many substances can trigger hives, including:

Animal dander (especially cats)

Insect bites

Medicines

Pollen

Shellfish, fish, nuts, eggs, milk, and other foods

Hives may also develop as a result of:

Emotional stress

Extreme cold or sun exposure

Excessive perspiration

Illness, including lupus, other autoimmune diseases, and leukemia

Infections such as mononucleosis

Exercise

Exposure to water

Often, the cause of hives is not known.

Symptoms

Symptoms of hives may include any of the following:

Itching

Swelling of the surface of the skin into red- or skin-colored welts (called wheals) with clearly defined edges.

Wheals may get bigger, spread, and join together to form larger areas of flat, raised skin.

Wheals can also change shape, disappear, and reappear within minutes or hours.You know you have hives when you press the center of a wheal, it turns white. This is called blanching.

Dermatographism is a type of hives. It is caused by pressure on the skin and results in immediate hives.

Exams and Tests

Your health care provider can tell if you have hives by looking at your skin.

If you have a history of an allergy, the diagnosis is even clearer.

Sometimes, a skin biopsy or blood tests are done to confirm that you had an allergic reaction, and to test for the substance that caused the allergic response.

Treatment

Treatment may not be needed if the hives are mild. They may disappear on their own. To reduce itching and swelling:

Do not take hot baths or showers.

Do not wear tight-fitting clothing, which can irritate the area.

Your provider may suggest that you take an antihistamine such as diphenhydramine (Benadryl). Follow your provider's instructions or the package instructions about how to take the medicine.

If your reaction is severe, especially if the swelling involves your throat, you may need an emergency shot of epinephrine (adrenaline) or steroids. Hives in the throat can block your airway, making it difficult to breathe.

Outlook (Prognosis)

Hives may be uncomfortable, but they are usually harmless and disappear on their own. In most cases, the exact cause of hives cannot be identified.

When the condition lasts longer than 6 weeks, it is called chronic hives. Often, no cause can be found.

Possible Complications

Anaphylaxis (a life-threatening, whole-body allergic reaction that causes breathing difficulty)

Swelling in the throat can lead to life-threatening airway blockage

When to Contact a Medical Professional

Call 911 or your local emergency number if you have:

Fainting

Shortness of breath

Tightness in your throat

Tongue or face swelling