Hives is a usually harmless skin condition that is caused by an increased blood flow and fluid in the dermis, resulting in the appearance of red raised areas on the skin. Hives may vary from a few millimeters to several centimeters in diameter and can appear anywhere on the body.

Hives may be caused by allergic reactions, viral infections, and physical irritation, but, in most cases, the cause cannot be identified. In the example of an allergic reaction, an allergy occurs when the body reacts to foreign particles it can't tolerate. Allergens are normally harmless, but the immune system of a person with allergies views them as harmful. After the body’s first contact with an allergen, white blood cells produce antibodies that prepare the immune system for the same allergen the next time it enters the body. Antibodies attach themselves to mast cells. Later contact with even a small amount of the allergen triggers the mast cells to release histamine. When histamine is released, it attaches to nearby cells and causes blood vessels to dilate and release fluids, creating the appearance of hives. Regardless of the cause, all hives are a result of histamine release.

Symptoms commonly associated with hives include

Individual hives generally last 2 to 24 hours.

Treatments such as antihistamines and applying cool compresses may be used to reduce symptoms. Although there is no test available to determine the exact cause of hives, allergy skin tests may be performed to determine allergens to which you are allergic.