Hives occur when specialized immune system cells in the skin, called mast cells, activate. Activated mast cells release inflammatory chemicals, including histamine and others. This causes dilation of nearby blood vessels and leakage of fluid into the superficial skin layer, resulting in hives. Hives, also known as urticaria, develop quickly and appear as slightly raised, irregular pink or red patches that characteristically itch. Hives occur for a variety of reasons, including immunologic and nonimmunologic causes.
Allergies commonly cause of hives. An allergy-provoking substance, or allergen, interacts with matching antibodies on the surface of mast cells in the skin, triggering their activation and resulting in hives. Ingested allergens are common culprits. Foods, such as nuts, shellfish and eggs, can trigger hives in sensitive people. Oral medications, especially certain antibiotics, can also lead to hives in those with allergies. Other common culprits include bug bites and stings, and exposure to latex -- all of which involve direct exposure of the skin to allergens. Allergy-related hives usually clear up within a few hours to days. However, they may be accompanied by other symptoms, which sometimes progress to a severe allergic reaction called anaphylaxis that requires emergency medical treatment.
Infections rev up the immune system, which can lead to mast cell activation and hives. Infection-related hives affect children more commonly than adults. A wide variety of bacterial, viral and parasitic infections can trigger hives. However, upper respiratory and gastrointestinal infections are the most common culprits. Examples of infectious agents that might trigger hives include:
-- strep and staph bacteria
-- H. pylori bacteria, which is often present with stomach and intestinal ulcers
-- HIV, influenza and hepatitis viruses
-- norovirus and rotavirus, which commonly cause the stomach "flu"
-- Epstein-Barr virus, which causes infectious mononucleosis
Hives usually go away in a few days to weeks with short-term infections, such as the flu or strep throat. However, people with long-term infections, such as HIV or chronic viral hepatitis, may experience persistent or recurring hives.
Physical factors affecting the skin sometimes cause hives. Dermatographia, which translates to skin writing, is the most common type of physical urticaria. This manifests as development of a hive when the skin is scratched, mirroring the exact site and direction of the scratch. These hives, which occur most frequently in young adults, are typically harmless and disappear within about 1 to 2 hours.
Cold urticaria describes the development of hives associated with exposure to cold temperatures. These hives usually develop quickly and go away within a few hours. It is unclear why cold exposure triggers activation of mast cells in some people but not others. Rarely, cold urticaria signals an underlying medical condition, such as a chronic infection. Other causes of physical hives include:
-- overheating, such as after a hot bath or strenuous workout
-- pressure on the skin, often due to tight clothing or jewelry
-- vibration, such as occurs with use of certain handheld tools
-- sun or water exposure, although this is very rare
Some medications and substances can cause direct mast cell activation and hives in sensitive people. These nonallergic hives can occur with radiocontrast dye used for some medical imaging tests; narcotic pain medications, such as morphine and hydrocodone; and the antibiotic vancomycin. Nonsteroidal anti-inflammatory drugs, such as aspirin and ibuprofen (Advil, Motrin) can cause hives by altering the metabolism of certain immune system chemicals.
Noninfectious illnesses also sometimes cause hives. People with autoimmune diseases, such as systemic lupus erythematosus and Hashimoto thyroiditis, can experience chronic hives. Hives might also occur with certain types of cancer, such as lung or colon cancer, and lymphoma. Less concerning are transient stress-related hives, which are thought to occur due a surge of adrenaline -- the so-called fight or flight hormone -- leading to mast cell activation. High stress might also be a factor in people with chronic hives.
Warnings and Precautions
Transient hives that are not accompanied by other symptoms typically do not indicate a serious underlying medical condition. However, if you experience frequent, persistent or recurring hives, see your doctor to determine the cause and best treatment. Seek emergency medical care if you break out in hives and have other symptoms that might indicate a serious allergic reaction, including:
-- swelling of the face, lips or tongue
-- wheezing, difficulty breathing or a change in your voice
-- lightheadedness, dizziness or weakness
-- nausea, with or without vomiting
-- anxiety, agitation or mental confusion
Reviewed and revised by: Tina M. St. John, M.D.