Hives is an allergic skin reaction causing localized redness, swelling, and itching.

Hives is a reaction of the body's immune system that causes areas of the skinto swell, itch, and become reddened (wheals). When the reaction is limited to small areas of the skin, it is called "urticaria." Involvement of larger areas, such as whole sections of a limb, is called "angioedema."

Hives is an allergic reaction. The body's immune system is normally responsible for protection from foreign invaders. When it becomes sensitized tonormally harmless substances, the resulting reaction is called an allergy. An attack of hives is set off when such a substance, called an allergen, is ingested, inhaled, or otherwise contacted. It interacts with immune cells called mast cells, which reside in the skin, airways, and digestive system.When mast cells encounter an allergen, they release histamine and other chemicals, both locally and into the bloodstream. These chemicals cause blood vessels to become more porous, allowing fluid to accumulate in tissue and leading to the swollen and reddish appearance of hives. Some of the chemicals released sensitize pain nerve endings, causing the affected area to become itchy and sensitive.

A wide variety of substances may cause hives in sensitive people, including foods, drugs, and insect bites or stings. Common culprits include:

  • Nuts, especially peanuts, walnuts, and Brazil nuts
  • Fish, mollusks, and shellfish
  • Eggs
  • Wheat
  • Milk
  • Strawberries
  • Food additives and preservatives
  • Penicillin or other antibiotics
  • Flu vaccines
  • Tetanus toxoid vaccine
  • Gamma globulin
  • Bee, wasp, and hornet stings
  • Bites of mosquitoes, fleas, and scabies.

Urticaria is characterized by redness, swelling, and itching of small areas of the skin. These patches usually grow and recede in less than a day, but maybe replaced by others in other locations. Angioedema is characterized by more diffuse swelling. Swelling of the airways may cause wheezing and respiratory distress. In severe cases, airway obstruction may occur.

Hives are easily diagnosed by visual inspection. The cause of hives is usually apparent, but may require a careful medical history in some cases.

Mild cases of hives are treated with antihistamines, such as diphenhydramine(Benadryl). An oatmeal bath may also relieve itching. More severe cases may require oral corticosteroids, such as prednisone. Topical corticosteroids arenot effective. Airway swelling may require emergency injection of epinephrine(adrenaline).

Most cases of hives clear up within one to seven days without treatment, providing the cause (allergen) is found and avoided.

Preventing hives depends on avoiding the allergen causing them. Analysis of new items in the diet or new drugs taken may reveal the likely source of the reaction. Chronic hives may be aggravated by stress, caffeine, alcohol, or tobacco; avoiding these may reduce the frequency of reactions.

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