The Hygiene Hypothesis or Old Friends Hypothesis

In support of the hygiene hypothesis there is evidence that allergic disorders and some autoimmune diseases are accompanied by a failure of the regulatory mechanisms that should terminate inflammatory responses. The situation is less clear in IBD but defective regulatory pathways are the key etiological factor in animal models of IBD, and tolerance to gut contents is broken in human IBD.

The old friends hypothesis suggests that the lack of appropriate levels of immunoregulatory pathways in developed countries is a consequence of diminished exposure to two categories of organism. First, harmless organisms associated with soil, untreated water and fermenting vegetable matter, and second, helminth infections (parasitic worms) that are still common in developing countries but almost completely absent from developed ones.

The former need to be tolerated because they are harmless and were present in food and water throughout human evolution. The helminth parasites need to be tolerated because, although not always harmless, once they are established in the host any effort by the immune system to eliminate them is likely to cause tissue damage.

For instance, a futile effort to destroy Brugia malayi microfilariae results in lymphatic blockage and elephantiasis.


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