Today, out of the blue, my allergies are going haywire. Its hard to get anything done when you have to wipe your nose or sneeze every few minutes, never mind the constant headache. Very annoying to say the least. Recently I was listening to a Radiolab podcast about parasites (which you can find here) and heard a segment about hookworms. “How do hookworms relate to allergies?”, you may ask. Some people think that one reason for the prevalence of allergies in developed nations is that we are too clean. Well……. sort of. The idea is that we aren’t typically exposed to several types of infectious agents, like parasites, which is a good thing. However, those infectious agents activate a certain part of our immune system that in turn depresses the part of out immune system who’s hyperactivity is responsible for allergies. Its called the hygeine hypothesis:
According to the hygiene hypothesis, proposed by David P. Strachan, allergic diseases are caused by inappropriate immunological responses to harmless antigens driven by a TH2-mediated immune response. Many bacteria and viruses elicit a TH1-mediated immune response, which down-regulates TH2 responses. The first proposed mechanism of action of the hygiene hypothesis stated that insufficient stimulation of the TH1 arm of the immune system lead to an overactive TH2 arm, which in turn led to allergic disease. In other words, individuals living in too sterile an environment are not exposed to enough pathogens to keep the immune system busy. Since our bodies evolved to deal with a certain level of such pathogens, when it is not exposed to this level the immune system will attack harmless antigens, and thus normally benign microbial objects, like pollen, will trigger an immune response. 
The hygiene hypothesis was developed to explain the observation that hay fever and eczema, both allergic diseases, were less common in children from larger families, which were presumably exposed to more infectious agents through their siblings, than in children from families with only one child. The hygiene hypothesis has been extensively investigated by immunologists and epidemiologists and has become an important theoretical framework for the study of allergic disorders. It is used to explain the increase in allergic diseases that has been seen since industrialization, and the higher incidence of allergic diseases in more developed countries. The hygiene hypothesis has now expanded to include exposure to symbiotic bacteria and parasites as important modulators of immune system development, along with infectious agents.
Epidemiological data support the hygiene hypothesis. Studies have shown that various immunological and autoimmune diseases are much less common in the developing world than the industrialized world and that immigrants to the industrialized world from the developing world increasingly develop immunological disorders in relation to the length of time since arrival in the industrialized world. Longitudinal studies in the third world demonstrate an increase in immunological disorders as a country grows more affluent and, presumably, cleaner. The use of antibiotics in the first year of life has been linked to asthma and other allergic diseases. The use of antibacterial cleaning products has also been associated with higher incidence of asthma, as has birth by Caesarean section rather than vaginal birth.
I knew allergies were caused by a hyperactive immune system, but I never really considered this explanation. Not sure what I think, but it make some sense. Thoughts?