Don't know why, but I like health topics :)
I've been meaning to write about this one for a while. It may not really be of interest to any casual readers, but could be useful for anyone who visits when googling this term, etc.!
The words "urticaria pigmentosa" literally mean "pigmented hives". However, this skin condition is not hives, but one of the several forms that mastocytosis can take. Mastocytosis is the name for problems involving abnormal clumping of mast cells in the body -- they are the cells that produce histamine, a chemical that is produced by the body during an allergic reaction. As far as I know, though, mastocytosis does not cause allergies -- however if a child has both allergies and mastocytosis, then they will have extra problems. Also, mastocytosis can react to a wide variety of triggers, and can cause problems not normally associated with allergies.
My younger son (now age 10) - here called "Jiji", has/had a mild case of this problem. It is pretty rare, enough that many doctors have not heard of it (except for dermatologists, of course). It may effect somewhere between 1 to 6 /10,000 people (according to the internet - a few sites estimate 1/10,000, and http://www.mastokids.org/ has a video which estimates there are less than 200,000 people with it in the U.S., which would be less than 6.7 people/10,000).
Urticaria Pigmentosa (I'll abbreviate it as UP) is the form of mastocytosis where the mast cells form little clumps here and there in the skin, and for some reason these clumps draw in melanin, leading to the formation of brown spots which are semi-permanent (they take several years to fade, after the problem has subsided). UP is often found in children, and has a good chance of going away by adolescence.