Urticaria related to Angioedema

My son woke up with a large rash on the outside of his right thigh and across his lower back / buttocks.  Diagnosis was urticaria (hives) related to Angioedema.  Hives can occur when histamine and other inflammatory chemicals are released from white blood cells (mast cells) in the skin.  These white blood cells are the same ones involved in allergic reactions in the nose and eyes (hayfever).   Mast cells are a part of the immune system and can be found throughout the body. Inside the mast cell are tiny granules containing different chemicals that cause inflammation. Related to skin diseases, mast cells play an important role in hives and facial flushing caused by mastocytosis.  A cause for chronic urticaria is found in less than 5% of cases. Topical therapies for hives are available but generally ineffective. Allergic reactions are not known to cause chronic urticaria and allergy tests are usually not helpful.  Hives have various looks.  The ones my son has look similar to those in the image found online below.  He has been taking 1 tsp (5mL) Zyrtec daily for the past few weeks for the swelling and due to this recent hives breakout we are taking 2 tsp (10mL) daily (1 tsp in the morning, 1 tsp at night).  

There are other rashes that may look like hives but the fact that they remain stable and do not resolve within 24 hours is helpful in distinguishing them from hives. Such rashes may need to have a small specimen of skin removed and examined under the microscope (biopsy) to accurately determine the nature of the skin disease. 

TIP:  Take a picture of the rash to show the doctor.  If the appearance changes take multiple pictures.  It can help in diagnosis.