Hives, also known as urticaria, is an outbreak of swollen, pale red bumps or plaques (wheals) on the skin that appear suddenly—either as a result of the body’s reaction to certain allergens, or for unknown reasons.
Hives usually cause itching, but may also burn or sting. They can appear anywhere on the body, including the face, lips, tongue, throat, or ears. Hives vary in size (from a pencil eraser to a dinner plate), and may join together to form larger areas known as plaques. They can last for hours, or up to one day before fading.
Angioedema is similar to hives, but the swelling occurs beneath the skin instead of on the surface. Angioedema is characterized by deep swelling around the eyes and lips and sometimes of the genitals, hands, and feet. It generally lasts longer than hives, but the swelling usually goes away in less than 24 hours.
Rarely, angioedema of the throat, tongue, or lungs can block the airways, causing difficulty breathing. This may become life threatening.
Allergic hives and angioedema form when, in response to histamine, blood plasma leaks out of small blood vessels in the skin. Histamine is a chemical released from specialized cells along the skin’s blood vessels.
Allergic reactions, chemicals in certain foods, insect stings, sunlight exposure, or medications can all cause histamine release. It’s often impossible to find out exactly why hives have formed.