Hives are raised, often itchy, red welts on the surface of the skin. They are usually an allergic reaction to food or medicine.

When you have an allergic reaction to a substance, your body releases histamine and other chemicals into your bloodstream. This causes itching, swelling, and other symptoms. Hives are a common reaction, especially in people with other allergies such as hay fever.

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

Many substances can trigger hives, including:

• Animal dander (especially cats)
• Insect bites
• Medications
• 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

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

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

They can also change shape, disappear, and reappear within minutes or hours.

The welts tend to start suddenly and go away quickly. When you press the center of a red welt, it turns white. This is called blanching.

Your doctor can tell if you have hives by looking at your skin.

If you have a history of an allergy, then the diagnosis is even more obvious.

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

• Avoid hot baths or showers.
• Avoid irritating the area with tight-fitting clothing.
• Take antihistamines. Diphenhydramine is considered the most effective.

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

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

• Anaphylaxis (a life-threatening, whole-body allergic reaction that causes breathing difficulty)
• Swelling in the throat can lead to life-threatening airway blockage

Call 911 or your local emergency number if you have:

• Fainting
• Shortness of breath
• Tightness in your throat
• Tongue or face swelling
• Wheezing

Call your health care provider if the hives are severe, uncomfortable, and do not respond to self-care measures.

• Avoid exposure to substances that give you allergic reactions.
• Don't wear tight-fitting clothing and avoid hot baths or showers just after an episode of hives. These can both cause the hives to return.

Amar SM. Uticaria. Prim Care. March 1, 2008; 35(1): 141-57, vii-viii.

Habif TP. Clinical Dermatology. 4th ed. St. Louis, Mo: Mosby; 2004.