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After introduction of antibiotics in the 1940s, the number of infectious disease in the developed world went down sharply. People started to live longer and healthier. But a number of rather unexpected and unpleasant effects were also observed. One of them is a gradual and steady rise in autoimmune diseases and allergic reactions. Medical scientists and researchers do not have a clear explanation to this phenomenon at present time. Several interesting explanations were suggested, but none of them is proven beyond reasonable doubts by statistical and epidemiological data.
This hypothesis first came to light when David Strachan, in an article published in 2000, observed an inverse correlation between the occurrence of hay fever and the number of older siblings, after following more than 17 thousand children since they were born, back in 1958.
Family size, hygiene and infection diseases
Strachan’s hypothesis is centered on the possibility that, in smaller families, microbial exposure is limited, because the risk of infections spreading person to person is also smaller. However, Strachan also concedes that part of the reduction in infections in recent years is caused by improved hygiene standards, household conditions and general personal cleanliness.
But how do exactly less infections equal more allergic conditions? As a matter of fact, there is some biologic basis for this hypothesis. Studies conducted with various types of bacteria and viruses show that infection with these microbial agents activates so-called TH1-mediated immune response, which down regulates another type of immune reactions called TH2-mediated responses. These TH2-mediated responses are the ones usually elicited in response to absolutely normal antigens (various chemicals and microbes in our environment) and which originate allergic conditions and allergies themselves.
This does support the hygiene hypothesis to some extent, as inadequate stimulation of the TH1 response might have led to overactive TH2 response, and consequently to a very sensitive antibody-mediated immunity. The net result of such combination of factors can be allergic disorders. Nevertheless, this explanation does not stand on its own, because the rise in incidence of allergic diseases was accompanied by a rise in autoimmune diseases, which are mediated by TH1 responses of the immune system.
Lack of exposure to infectious agents equals weak immune system
An alternative explanation has thus arisen. Based on a series of experimental investigations, researchers have hypothesized that, since regulatory T cells of our immune system need to be stimulated by infectious agents (be them bacteria, viruses or parasites) in order to control TH-mediated responses, the absence of such stimulation would lead to poorly repressed TH1 and TH2 responses. A rise in autoimmune and allergic disorders follows from this theory.
Besides immunologic explanations, the hygiene hypothesis is strongly supported by epidemiological data.