A reader writes:
I'm a microbiology and immunology graduate student at Johns Hopkins, and I thought I'd add a bit to this post. The hygiene hypothesis has been developed quite a bit, and its characterization in the post is overly simplistic. Rather than focusing on exposure to "dirt", the key really seems to be the lack of intestinal helminth (worm) infections in the developed world. I don't want to get overly technical, but these organisms chronically infect people. In fact, in the developing world almost everyone is infected by one or more species of helminth at all times. These infections induce an immune response that suppresses the type of immune response that is responsible for allergy, which is due to Type 1 Hypersensitivity Reactions.
Another expert weighs in:
First, a one line CV: I'm a biologist at the University of Chicago (my focus is neurobiology but I have a strong general background). I think the hygiene hypothesis is correct but does not tell the whole story. In addition to food allergies, allergies in general are going up at an alarming rate. While the lack of exposure to environmental bacteria and the like certainly plays a role, let's not forget what kids in the city are exposed to. Cities are chock full of toxic agents; the very air we breath is dirty and bad for us, especially for the young.
Here in Chicago, century-old coal plants are just now being shutdown (a fight I am proud to say I played a part in as a local activist). Still, the contamination of the water, land, and air is long-lasting and has hugely important impacts on child allergies. In Pilsen, a neighborhood one of those plants is located in, if you left a window open, particulate matter would settle on your apartment floor. Truly horrible and more impactful on children's health than Purell, which isn't really that effective anyway.
The hygiene hypothesis is probably true but let's keep our eyes on the prize here: we city dwellers are constantly being poisoned.
The Dish 