Hives (urticaria) is an allergic reaction in the skin characterized
by white or pink welts or large bumps surrounded by redness.
These welts are known as wheal and flare lesions and are caused primarily by the release
of histamine (an allergy mediator) in the skin. About 50% of people with chronic hives develop
angioedema—a deeper, more serious form of hives involving the tissue below the surface of the skin.
While the basic cause of hives involves the release of histamine from white blood cells, what actually
triggers this release can be a variety of factors, such as physical contact or pressure, heat (prickly heat
rash), cold, water, autoimmune reactions, infectious organisms (e.g.,
hepatitis B virus,
Candida albicans, and streptococcal bacteria), and allergies or sensitivities to drugs (especially antibiotics and aspirin), foods, and food additives.