MODEL RELEASED. Urticaria rash due to vaccine reaction on the skin of the abdomen of a 7-year-old boy. Urticaria (also called hives) is a response of the body's immune system, usually to an allergen, stress or a drug. The rash is itchy and painful. Treatment is to avoid the underlying cause, if known. Here, the cause was use of the MMR (measles, mumps and rubella) vaccine.