Each bump lasts a few hours, before fading with no lasting mark, only for some new ones to appear. Any part of the body may be affected. When very soft tissues, such as around the eyes or mouth, are affected there can be immense swelling, which can be frightening, but which usually settle quite swiftly. The trigger for hives is not clear, but you may be able to pinpoint a particular allergy or chemical cause. One common cause is food, especially prawns and seafood, nuts, tomatoes and eggs.
Some medicines also contain chemical colourings or preservatives that can trigger urticaria.
Other triggers include heat, cold, and pressure on the skin. Many infections can cause hives, and colds are a common cause in children. Histamine release causes swelling Underneath the skin, these triggers cause the release of a chemical called histamine from the cells that line the blood vessels. Histamine makes the blood vessels leaky, causing fluid to seep out into the tissues, and hence swelling to occur. The reaction can carry on from anything up to a few hours or days, to several weeks.