Excerpt from Natural Baby and Childcare

Hives, also known as welts, urticaria, or nettle-rash, occur when a substance called histamine is released in the body.  Histamine works as a defense mechanism when it comes into contact with something foreign, causing the capillaries (small blood vessels) to dilate and form welts on the surface of the skin.  Welts are warm, circular, raised, and can be intensely itchy. They look like a mosquito bite, red on the outside and white in the center. The rash often appears in clusters and can come and go lasting from minutes to days.  Urticaria is usually the result of an acute allergic reaction from drugs, food, pollen, and occur after exercise and from being in the cold. There are many foods to which your child may be allergic but some of the most common are nuts (especially peanuts), dairy, eggs, and shellfish. Hives can also result from chronic infections, autoimmune diseases, cancer, or other causes. Severe life-threatening swelling of mouth and throat requires immediate emergency medical care. 

In addition, some children who are susceptible to hives suffer severe anaphylactic reactions to allergenic substances.  As a precaution, parents keep an emergency  injectable epinephrine  kit (EpiPen) to be given to children who have a history of life threatening reactions.. Parents should be instructed on how to use it ahead of time.