The Hygiene Hypothesis or Old Friends Hypothesis

The hygiene hypothesis proposes that several chronic inflammatory disorders (allergies, autoimmunity, inflammatory bowel disease) are increasing in prevalence in developed countries because a changing microbial environment has perturbed immunoregulatory circuits which normally terminate inflammatory responses. This effect is so prevalent that even disorders commonly not considered immune related, such as some stress-related psychiatric disorders, particularly depression and anxiety, are associated with markers of ongoing inflammation, even without any accompanying inflammatory disorder. It is probably no accident that some of the modern anti-depressants are known to have neuro-anti-inflammatory properties. Moreover, pro-inflammatory cytokines can induce depression, which is commonly seen in patients treated with interleukin-2 or interferon-a. Therefore, some psychiatric disorders in developed countries might be attributable to failure of immunoregulatory circuits to terminate ongoing inflammatory responses.

If one examines the epidiemiology of heart disease one finds an unexplained pro-inflammatory process at work. It is tempting to attribute it to the absence of benign infectious organisms and their anti-inflammatory immune regulatory properties.

The hygiene hypothesis holds enormous potential to explain almost every, if not every, "modern" disease involving immune dysregulation or inflammation.