Because somebody asked...
The rise we are seeing in allergies is primarily witnessed in the first world. Studies indicate that a staggering 40% of the developed world's population is suffering from some sort of allergy (ie. mounting of IgE response). If you mount an exaggerated IgE response you are considered to be atopic, and such individuals are usually characterized by a higher IgE blood content than non-atopic people. This is a genetic factor, and there are a number of candidate genes being looked at on I believe 2 separate chromosomes, but I'd have to look that up.
There are 4 theories as to the increased allergic responses:
1. Dietary changes (no evidence to prove this).
2. Increased allergen levels in your environement (no evidence).
3. Environmental pollution - this one is a popular one, especially in the natural medicine community, but most people would be surprised to find out that the strongest evidence actually shows the oppposite is true. There is a paper by von Mutius et al. (1994) which followed two cohorts of children in Germany. One was in the heavily industrialized, polluted town of Halle in the former East Germany, with severe levels of air pollutants. The other cohort was an ethnically matched (to avoid genetic tendency for atopy among different ethnicities) group in Munich, where the air was considerably cleaner. The results were that the children of Halle had a significantly lower incidence of atopy than the children in Munich! It should be noted that the effects of air pollution were obvious on them, as their rate of respiratory disease was considerably higher than that in Munich. But alas, environmental pollution not only did not induce an atopic response, but to some extent seems to have suppressed it.
4. The most believable explanation is the changes in exposure to pathogens in early childhood. The human body has two subsets of T-helper cells, known as Th1 and Th2. Th1 cells are most commonly associated with defense against intracellular parasites (and some autoimmune responses). Th2 cells are associated with atopic and anti-inflammatory responses. The current theory is that the first world has largely removed parasitic infestation and infection through clean drinking water and sanitary living conditions. This may have resulted in skewing our Tcell response in favour of Th2 cells so that we are getting exaggerated IgE responses, atopy and allergy. People in the third world do not share the same high incidence of allergies, but notably they do have high rates of parasitic infections. There are therefore studies being done regarding how to potentiall equalize the T helper cell responses so it is not skewed in one direction.