In medicine, the hygiene hypothesis states that
a lack of early childhood exposure to infectious agents, symbiotic
microorganisms (e.g. gut flora), and parasites increases susceptibility to allergic
diseases by modulating immune system development
History
First proposed by David P. Strachan in an article published in the British
Medical Journal (now BMJ), in 1989, 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 (see Current Theories below).
Mechanism of Action
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 .
The first proposed mechanistic explanation for the hygiene hypothesis cannot
explain the rise in incidence (similar to the rise of allergic diseases) of
several TH1-mediated autoimmune diseases, including inflammatory bowel disease
(IBD), multiple sclerosis (MS), and type I diabetes. The major proposed
alternative mechanistic explanation is that the developing immune system must
receive stimuli (from infectious agents, symbiotic bacteria, or parasites) in
order to adequately develop regulatory T cells, or it will be more susceptible
to autoimmune diseases and allergic diseases, because of insufficiently
repressed TH1 and TH2 responses, respectively.. To use a rough analogy, an
unbridled immune system (without regulatory T cells) has the dynamic of a
rowdy, unchaperoned beer party. It is likely to overreact to slight or
non-existent insults (analogous to allergic disease) and may even attack
members of its own party (analogous to autoimmune disease). The role of the T
regulatory cells of the immune system is similar to that of the bouncer, keeping
the beer party in check.
Current Theories
The hygiene hypothesis has expanded to include exposure to several varieties
of microorganisms and parasites, which humans coexisted with throughout much of
our evolutionary history, as necessary for balanced and regulated immune system
development.
In recent times, the development of
hygienic practices and effective medical care have diminished or eliminated
exposure to these microorganisms and parasites during development.
Examples of organisms that may be important for proper development of T
regulatory cells include lactobacilli, various mycobacteriums, and certain helminthes.
Epidemiological data supports the hygiene hypothesis. 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, along with birth by Caesarean
section rather than vaginal birth.
According to the hygiene hypothesis,
the mechanistic explanation for these associations is that antibiotics,
antibacterial cleaning products, and C-sections limit necessary exposure to
symbiotic bacteria, infectious diseases, and parasites, and thus regulatory T
cells do not develop properly and the immune system is left unchecked.
Bottomline is to think before use of antiseptics like detol or phenyl or carbolic acid etc. in your house or on body ???.
Dr.Indresh
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