Histamine causes the blood vessels to dilate and leak fluid into the surrounding tissue causing the skin to swell. This reaction irritates nerve endings resulting in itching. In severe reactions, feeling of nausea, vomiting and dizziness may be present. The medical name for hives is urticaria. Any individual hive will last only a few hours and then fade, but new ones will occur.
Hives can occur anywhere on the body, but are especially common about the elbows and knees.
They are usually small, ranging from 1 to 3 cm. in size. However, giant hives can occur covering large areas, most commonly on the trunk. Release of histamine will sometimes cause swelling of the eyelids, lips and tongue. Hives are usually not dangerous, but swelling of the lips can be alarming, and may indicate a more severe allergic reaction. If the tongue swells or any difficulty in breathing occurs immediate medical attention should be sought.