Allergic rashes are also referred to as hives or urticaria. These rashes result from an allergic reaction to certain foods or medicines and are more common in people with other allergic disorders or those with a family history of allergy. They present as an itchy skin rash similar to that of an insect bite, with smooth raised reddish blebs known as wheals. They are more common over the arms and legs but may be present on other parts of the body as well. They may increase in size and merge with each other resulting in frequent changes in shape. They may also disappear and reappear at the same place. They characteristically exhibit the blanching phenomenon, wherein they turn pale when some pressure is applied at their center. In some severe cases the allergic rashes may also be accompanied by swelling on the lips, eyes, hands, feet and throat (angioedema). Angioedema may cause difficulty in breathing which may be life threatening.
Allergic rashes may last from a few days to months. Depending on this duration, they can be classified into acute allergic rashes, which may last up to 6 weeks, and chronic allergic rashes, lasting beyond 6 weeks.