The pathophysiology of the hygiene hypothesis

Full text

Review
The
pathophysiology
of
the
hygiene
hypothesis
Emmanuel
Prokopakis
a,
*,
Alexios
Vardouniotis
a
,
Hideyuki
Kawauchi
b
,
Glenis
Scadding
c
,
Christos
Georgalas
d
,
Peter
Hellings
e
,
George
Velegrakis
a
,
Livije
Kalogjera
f
a
Department
of
Otorhinolaryngology,
University
of
Crete
School
of
Medicine,
Crete,
Greece
b
Department
of
Otorhinolaryngology,
University
of
Shimane
School
of
Medicine,
Shimane,
Japan
c
Department
of
Allergy
and
Rhinology,
Royal
National
TNE
Hospital,
London,
Gr.
Britain,
UK
d
Endoscopic
Skull
Base
Centre,
Academic
Medical
Centre,
Amsterdam,
The
Netherlands
e
Department
of
Otorhinolaryngology,
Katholic
University
of
Leuven,
Leuven,
Belgium
f
Department
of
Otorhinolaryngology,
University
Hospital
Centre
‘‘Sestre
milosrdnice’’,
Zagreb,
Croatia
Contents
1.
Introduction
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2.
Hygiene
hypothesis
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3.
Pathophysiological
route
of
the
allergic
infection
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4.
The
toll-like
receptor
system
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5.
Epigenetics
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6.
Family
and
hygiene
hypothesis.
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7.
Endotoxins
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8.
Hygiene
hypothesis
and
auto-immune
diseases.
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9.
Antibiotics
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1068
10.
Microbial
and
virus
infections.
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1069
11.
‘‘Western’’
way
of
life
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1069
12.
Vitamin
D.
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1069
13.
Conclusion
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1070
References
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1.
Introduction
Allergy
as
known
is
considered
to
be
a
disturbance
of
the
immune
regulation,
but
the
exact
level
or
levels
of
this
malfunction
remain
until
today
partially
identified.
During
the
last
decades
as
the
number
of
patients
with
allergies
is
constantly
increasing,
several
theories
have
been
developed,
in
order
to
provide
a
International
Journal
of
Pediatric
Otorhinolaryngology
77
(2013)
1065–1071
A
R
T
I
C
L
E
I
N
F
O
Article
history:
Received
11
February
2013
Received
in
revised
form
24
April
2013
Accepted
27
April
2013
Available
online
20
May
2013
Keywords:
Hygiene
hypothesis
Allergy
Asthma
Allergic
rhinitis
Endotoxins
Epigenetics
A
B
S
T
R
A
C
T
There
has
been
a
considerable
increase
in
the
diagnosis
of
allergic
diseases
over
the
last
decades.
Prevalence
of
allergies
in
high-income
countries
and
urban
areas
appears
higher
than
in
rural
environments.
While
environmental
factors
like
pollution
or
nutrition
can
be
important,
it
is
more
likely
that
in
the
end
they
have
a
small
association
with
allergies.
Childhood
infections
and
exposure
to
certain
microbial
antigens
on
the
other
hand
seem
to
present
a
strong
negative
correlation
with
allergies,
and
therefore
the
increase
of
the
allergic
burden
in
the
Western
world
has
been
frequently
related
to
a
decline
of
childhood
infections
giving
birth
to
the
‘‘Hygiene
Hypothesis’’.
We
address
the
issue
with
emphasis
on
the
associated
pathophysiology
tightrope
walking
between
the
skepticism
of
the
critics,
which
cast
doubt
on
it,
and
the
pilgrims’
belief
of
having
discovered
allergy’s
Holy
Grail.
ß
2013
Elsevier
Ireland
Ltd.
All
rights
reserved.
*
Corresponding
author
at:
University
Hospital
of
Crete,
Building
A
3rd
Floor,
University
Avenue,
71110
Heraklion,
Crete,
Greece.
Tel.:
+30
6932
237622.
E-mail
address:
eprokopakis@gmail.com
(E.
Prokopakis).
Contents
lists
available
at
SciVerse
ScienceDirect
International
Journal
of
Pediatric
Otorhinolaryngology
jo
ur
n
al
ho
m
ep
ag
e:
ww
w.els
evier
.c
om
/lo
cat
e/ijp
o
r
l
0165-5876/$
–
see
front
matter
ß
2013
Elsevier
Ireland
Ltd.
All
rights
reserved.
http://dx.doi.org/10.1016/j.ijporl.2013.04.036

sufficient
explanation
of
this
phenomenon.
Going
back
to
the
early
60s
all
started
with
immunoglobin
E
and
its
relationship
to
allergy,
while
in
the
70s,
IgE
gave
its
place
to
mast
cells
and
eosinophiles,
succeeded
by
environmental
allergy
in
the
80s,
and
the
microbio-
logical
background
of
allergy
during
the
90s.
All
these
theories
had–to
one
point
or
another-
as
common
denominator
the
differentiation
of
T-cells
towards
Th1
or
Th2.
Hygiene
hypothesis
has
been
presented
as
the
rightful
heir
of
the
former
theories,
claiming
the
role
of
a
protagonist
in
the
allergy
series,
by
attempting
to
resolve
ongoing
issues,
and
answering
long
lasting
questions
about
allergy.
The
concept
at
first
focused
on
allergic
disorders,
but
since
in
the
last
decades
in
developed
countries
several
categories
of
chronic
inflammatory
disorders
have
become
much
more
preva-
lent,
it
extended
to
autoimmunity
and
inflammatory
diseases
as
well.
It
is
estimated
that
in
the
United
States
alone
fifty
million
patients
are
affected
from
some
kind
of
allergy,
such
as
allergic
rhinitis,
eczema
or
asthma,
and
that
eight
more
million
suffer
from
some
kind
of
self
immune
disease,
such
as
systemic
lupus
erythematosus,
rheumatoid
arthritis
and
insulin
depended
diabe-
tes.
Hygiene
hypothesis
presents
with
the
assumption
that
reduced
exposure
to
microorganisms
leads
to
a
disordered
regulation
of
the
immune
system
in
general
and
hence
in
a
raise
of
certain
inflammatory
disorders.
Based
on
the
fact
that
the
most
critical
period
in
the
development
of
the
immune
system
is
during
infancy,
any
kind
of
intervention
at
the
time,
as
for
instance
a
tonsillectomy,
could
alter
the
response
leading
to
increased
susceptibility
to
allergic
disease,
due
to
the
tonsils’
participation
in
the
defense
against
airborne
and
alimentary
micro
organisms
and
therefore
impaired
cellular
and
humoral
stimulation.
2.
Hygiene
hypothesis
The
initial
definition
of
Hygiene
Hypothesis
as
proposed
by
the
epidemiologist
D.
Strachan
in
1989
[1],
quoted:
‘‘The
apparent
rise
in
the
prevalence
of
allergic
diseases
could
be
explained
if
allergic
diseases
were
prevented
by
infection
in
early
childhood,
transmit-
ted
by
unhygienic
contact
with
older
siblings,
or
acquired
prenatally.
Over
the
past
century
declining
family
size,
improved
household
amenities
and
higher
standards
of
personal
cleanliness
have
reduced
opportunities
for
cross-infection
in
young
families.
This
may
have
resulted
in
more
widespread
clinical
expression
of
atopic
disease’’
His
research
collected
epidemiological
data
for
allergic
rhinitis
and
eczema
from
a
national
sample
of
17.414
children
from
schools
in
Great
Britain
during
a
period
from
their
birth–during
one
week
in
March
1958-
until
they
had
reached
the
age
of
23.
The
three
parameters
that
were
studied
were
the
patient’s
statement
of
allergy
at
the
age
of
23,
their
parents’
testimony
of
allergic
rhinitis
at
the
age
of
11
and
their
history
of
eczema
at
the
age
of
1,
as
stated
again
from
their
parents.
The
outcome
of
this
research
depicted
a
decrease
in
the
prevalence
of
allergic
rhinitis
and
eczema
as
the
number
of
older
children
in
the
family
increased
and
did
not
relate
to
the
socioeconomic
status
of
the
family.
In
order
to
explain
these
rather
unprecedented
findings,
Strachan
made
the
assumption
that
the
presence
of
older
children
or
generally
siblings
in
a
large
family
had
a
protective
effect
against
allergic
sensitization.
Initially,
the
hypothesis
was
based
on
epidemiological
evidence
and
the
obvious
at
the
time
explanation
was
the
larger
amount
of
infections
in
these
families
that
seemed
to
somehow
lower
the
manifestation
of
atopy.
Hygiene
hypothesis
has
been
since
then
supported
from
a
large
number
of
studies
that
also
proved
the
beneficiary
effect
of
having
older
children
in
the
family,
as
well
as
in
children
attending
day
care
units,
especially
during
the
first
months
of
life,
resulting
in
less
patients
with
allergic
rhinitis,
asthma
or
other
allergic
diseases
[2,3].
The
new
theory
was
greeted
with
enthusiasm
and
a
rapid
uptake
over
the
years
to
follow
and
new
evidence
from
epidemiological,
biological
and
genetic
studies
significantly
enlarged
the
scope
of
the
hypothesis.
However,
more
recent
research
has
come
up
with
contradictory
evidence
that
argues
against
certain
facts
attributed
to
hygiene’s
basic
points.
In
contrast
to
what
would
be
expected–in
a
survey
conducted
in
children
in
Australia
between
2000
and
2005-
asthma
prevalence
has
declined
whereas
eczema
prevalence
has
increased.
Further-
more,
immigrant
and
non-immigrant
children
showed
a
similar
asthma
trend,
which
is
opposite
to
the
statement
that
immuno-
logical
responsiveness
during
childhood
is
established
within
the
first
year
of
life
or
even
earlier,
as
proposed
by
hygiene
hypothesis
[4].
3.
Pathophysiological
route
of
the
allergic
infection
It
is
necessary
to
mention
some
facts
that
are
highly
related
with
hygiene
hypothesis,
and
the
regulatory
role
of
T-cells
through
the
production
of
cytokines.
The
regulation
of
the
allergic
reaction
is
performed
by
the
interaction
between
cells
and
chemical
transmitters.
The
initial
exposure
to
an
allergic
antigen
produces
antibodies
IgE
from
B-cell
lymphocytes,
which
in
consequence
provokes
an
allergic
reaction
to
every
new
exposure
to
the
specific
antigen
that
is
regulated
by
T-cell
lymphocytes.
The
antigen
after
entering
the
human
body
is
captured
by
macrophages
and
dendritic
cells
and
presented
to
T-helpers
cells
(Th
0
).
Th
0
cells
have
the
ability
of
differentiating
either
to
Th
1
type
that
participate
in
immunological
reactions
by
inducing
macrophages
and
creating
inflammatory
tissue,
or
to
Th
2
type
that
induce
atopy
and
increase
the
production
of
IgE,
mast
cells
and
eosinophiles
participating
in
allergic
reactions
[5–8].
The
absence
of
balance
between
these
two
types
of
cells
seems
to
be
the
cause
of
various
diseases,
and
constitutes
the
basis
of
the
hygiene
hypothesis
theory.
The
immunological
definition
of
this
theory
came
afterwards
and
is
based
on
the
fact
that
children
coming
from
multimember
families
have
increased
possibilities
of
infections,
which
lead
their
immune
system
mainly
towards
the
direction
of
Th
1
cell
type
differentiation,
in
order
to
confront
infections
from
viruses
and
microorganisms.
On
the
other
hand,
children
growing
up
in
small
families,
facing
less
inflammatory
conditions
in
comparison,
develop
a
larger
number
of
Th
2
cells
instead
of
Th
1
,
which
leads
to
an
increased
development
of
allergic
responses
[9].
After
the
initial
sensitization,
the
differentiation
of
primitive
T-
helper
cells
to
Th
1
or
Th
2
depends
on
the
presence
of
interleukins
IL-12
and
IL-4
correspondently.
Th
2
lymphocytes
produce
a
number
of
interleukins
including
IL-4
and
IL-13
that
provoke
proliferation
of
B-cell
lymphocytes.
B-cell
lymphocytes
produce
antibodies
that
attach
to
organ-tissue
‘‘targets’’
and
when
exposed
to
the
antigen
for
the
second
time,
bond
with
it
and
begin
the
immediate
phase
of
allergic
reaction
with
the
degranulation
of
mast
cells.
Other
cytokines
like
IL-3
and
IL-5
have
a
similar
effect
on
the
proliferation
and
stimulating
of
eosinophiles
that
determine
the
delayed
phase
of
the
allergic
reaction
[10,11].
Additionally,
Th
1
lymphocytes
produce
IFN-
g
and
IL-2
that
participate
in
immune
reactions
like
sarcoidosis,
Chron’s
disease
etc.
Furthermore
IL-4
inhibits
the
expression
of
the
Th
1
lymphocytes,
while
IFN-
g
inhibits
the
expression
of
Th
2
lymphocytes
respectively.
However,
Th-2
hypothesis
for
asthma
based
on
experimental
studies
in
mice,
where
Th-1/Th2
polarization
is
clear,
may
be
too
simple
to
directly
imply
on
human
airway
disease,
where
interaction
of
Th-1
and
Th-
2
profile
cytokines
may
both
play
a
role
in
the
severity
of
the
disease
[12].
It
came
clear
more
than
a
decade
ago
that
application
E.
Prokopakis
et
al.
/
International
Journal
of
Pediatric
Otorhinolaryngology
77
(2013)
1065–1071
1066

of
biological
agents,
like
anti-Il-5,
and
Il-12,
shift
towards
Th-1
response,
although
decreasing
circulating
eosinophiles,
did
not
prevent
early
and
late
allergic
response
or
bronchial
reactivity
[13,14].
4.
The
toll-like
receptor
system
The
innate
immune
system
is
a
pivotal
defense
mechanism
in
humans.
Even
though
its
function
is
based
only
on
a
limited
number
of
receptors
–
expressed
on
the
surface
of
and
within
cells
–
it
is
capable
of
recognizing
an
abundant
number
of
pathogen-
associated
molecular
patterns
(PAMPs),
and
initiating
immune
responses.
Major
components
of
these
innate
immunity
pathogen
recognition
receptors
are
the
toll-like
receptors
(TLRs).
Backed
up
by
hygiene
hypothesis,
which
suggests
that
allergies
appear
because
of
a
change
in
microbial
exposure
and
associated
immune
signals
early
in
life,
it
has
been
speculated
that
alterations
in
TLRs
signaling
could
influence
allergy
development.
Therefore,
besides
their
role
in
inflammation
and
infection,
TLR
genes,
and
their
genetic
variations
as
well
as
their
association
with
allergy
and
atopic
diseases
were
investigated
over
the
past
years.
In
humans,
11
TLRs
have
been
identified
[15,16].
With
regard
to
their
cellular
localization,
TLRs
can
be
divided
into
two
subgroups:
TLR1,
TLR2,
TLR4,
TLR5,
TLR6,
TLR10,
and
TLR11
are
expressed
on
the
cell
surface,
while
TLR3,
TLR7,
TLR8,
and
TLR9
are
localized
in
intracellular
vesicles.
TLR2
is
involved
in
the
recognition
of
a
wide
range
of
PAMPs
derived
from
bacteria,
fungi,
parasites,
and
viruses.
It
generally
forms
a
heterodimer
with
TLR1
and
TLR6;
TLR1-
TLR2
respond
to
the
bacterial
triacylated
lipopeptide,
whereas
TLR2-
TLR6
recognize
the
mycobacterial
diacylated
lipopeptide.
TLR4
is
essential
for
responses
to
lipopolysaccharides
(LPS),
a
major
constituent
of
the
outer
membrane
of
Gram-negative
bacteria.
TLR5
recognizes
flagellin,
a
protein
component
of
bacterial
flagella.
TLR3
responds
to
viral
double-stranded
RNA,
while
TLR7
and
TLR8
preferentially
recognize
single-stranded
RNA.
TLR9
was
originally
identified
to
respond
to
specific
DNA
motifs
that
are
frequently
present
in
bacteria
[17].
The
knowledge
regarding
TLR10
and
11
ligands
is
still
incomplete,
even
though
recent
studies
have
demonstrated
that
TLR10
shares
some
common
agonists
with
TLR1
(and
TLR2)
[18].
Ligand
binding
on
TLRs
leads
to
an
activation
of
several
intracellular
signaling
that
induces
expression
of
costimulatory
molecules
(CD80
and
CD86)
and
proinflammatory
cytokines
(TNFa,
type-1
interferons,
IL-1,
IL-6,
IL-10,
and
IL-12)
that
mainly
favors
Th1
differentiation
[19–22].
According
to
the
hygiene
hypothesis,
TLRs
need
to
be
stimulated
in
the
early
period
of
life
to
drive
the
Th-cell
populations
from
the
in
utero-shaped
Th2-bias
into
a
Th1-
direction
[23].
Some
experiments,
mainly
in
animal
models
of
experimental
asthma,
showed
pronounced
Th2
responses
and
elevated
allergic
parameters
as
a
reaction
on
LPS
application
[24,25]
whereas
others
reported
reduced
asthma
phenotypes
after
LPS
application
[26,27].
However,
these
discrepancies
can
be
sufficiently
explained
by
dose-dependent
effects
[28].
In
the
ALEX
study
blood
cells
from
farmers’
children
expressed
significantly
higher
amounts
of
toll-like
receptor
2
than
those
from
non-
farmers’
children,
indicating
that
these
TLR
might
also
be
involved
in
the
‘‘farming
effect’’
[29].
TLRs
undoubtfully
play
an
important
role
in
the
prevention
in
allergic
disorders.
Furthermore,
there
are
certain
TLR
gene
polymorphisms
that
are
associated
with
an
increased
prevalence
of
allergic
diseases
[30].
For
example,
allergic
diseases
in
farmer’s
children
could
be
contributed
to
a
significantly
elevated
preva-
lence
of
a
polymorphism
found
in
the
TLR2-coding
gene
[31].
Results
from
a
Swedish
study
indicated
that
a
polymorphism
in
the
TLR4-gene
is
associated
with
asthma
characterized
by
a
decreased
IL-12
production
by
antigen
presenting
cells
(APCs)
after
LPS
stimulation
[32].
In
the
GABRIEL
Advanced
Studies)
[33]
conducted
in
rural
regions
of
Austria,
Germany,
and
Switzerland,
79,888
school-aged
children
categorized
as
‘‘farmers’’,
‘‘visiting
farms’’
and
‘‘non-
farmers’’,
answered
a
recruiting
questionnaire.
Afterwards
a
stratified
random
subsample
of
8419
children
answered
another
detailed
questionnaire
on
farming
environment.
A
broad
definition
of
asthma
comprising
symptoms,
diagnosis,
or
treatment
ever
was
used
and
blood
samples
and
specific
IgE
levels
were
available
for
7682
of
these
children.
The
conclusion
was
that
those
children
living
on
a
farm
were
at
significantly
reduced
risk
of
asthma,
hay
fever,
atopic
dermatitis,
and
atopic
sensitization
compared
to
children
visiting
farms
and
nonfarm
children
(asthma:
11%,
16%,
18%
respectively,
hay
fever:
5%,
11%,
15%
respectively).
It
seems
that
a
specific
type
of
traditional
farming
(i.e.,
with
cows
and
cultivation)
was
protective
against
asthma,
hay
fever,
and
atopy,
aligning
with
the
results
of
a
similar
study
[34]
comparing
the
prevalence
of
allergic
sensitization
in
a
population
of
Amish
children
(157
families)
to
children
both
living
in
farms
in
Switzerland
(3000
families)
and
nonfarm
children
(11,000
fami-
lies).
Based
on
parental
reporting,
the
prevalence
of
ever
having
asthma
was
5.2%,
6.8%,
and
11.3%
among
Amish,
Swiss
farm,
and
Swiss
nonfarm
children,
respectively.
The
corresponding
figures
for
allergic
sensitization
were
7.2%,
25.2%,
and
44.2%,
confirming
previous
observations.
Even
though,
in
the
GABRIEL
Studies
the
farm
effect
on
asthma
could
be
explained
by
specific
farm
characteristics,
there
is
still
a
link
missing
for
hay
fever
and
atopy,
whereas
in
the
Amish
population
study,
since
the
Amish
are
of
Swiss
descent
and
therefore
presenting
a
genetically
similar
background
to
Swiss
children,
the
exceedingly
low
level
of
sensitization
of
7.2%
among
Amish
children,
implicates
that
there
may
be
additional
protective
factors
in
this
population.
5.
Epigenetics
The
possible
link
between
all
the
above
observations
is
the
effect
of
environmental
factors
upon
gene
expression
in
man-.
Epigenetics
is
the
study
of
heritable
changes
in
gene
expression
or
cellular
phenotype
caused
by
mechanisms
other
than
changes
in
the
underlying
DNA
sequence–hence
the
name
epi-
(Greek:
epi
´-
over,
above,
outer)
-genetics
[35].
It
refers
to
functionally
relevant
modifications
to
the
genome
such
as
DNA
methylation
and
histone
modification,
that
do
not
involve
a
change
in
the
nucleotide
sequence
[35].
These
changes
may
remain
through
cell
divisions
for
the
remainder
of
the
cell’s
life
and
may
also
last
for
multiple
generations.
However,
there
is
no
change
in
the
underlying
DNA
sequence
of
the
organism;
instead,
non-genetic
factors
cause
the
organism’s
genes
to
behave
(or
‘‘express
themselves’’)
differently.
Recent
data
about
the
effect
of
diet
on
gene
methylation
offers
an
example
of
how
epigenetic
mechanisms
might
affect
gene-
environment
interactions
[36].
In
the
case
of
allergic
disease,
epigenetics
could
explain
the
discordances
observed
between
monozygous
twins
as
well
as
sporadic
cases,
incomplete
pene-
trance,
variable
expression,
gender
and
progenitor
effects.
The
hygiene
hypothesis
is
of
great
relevance
since
it
integrates
genetic
and
epidemiological
data
in
the
context
of
environmental
exposures
[36].
Environmental
factors
able
to
regulate
epigenetic
changes
include
diet:
folic
acid
and
vitamin
B12
influence
the
availability
of
methyl
groups
and
co-factors
for
the
formation
of
S-adenosyl-
methionine,
essential
for
de
novo
DNA
methylation.
Fish
oil-rich
diets
could
induce
epigenetic
effects.
Other
important
factors
are
the
microbial
composition
of
the
gut
flora
[37].
E.
Prokopakis
et
al.
/
International
Journal
of
Pediatric
Otorhinolaryngology
77
(2013)
1065–1071
1067

The
information
on
the
epigenetic
mechanism
possibly
underlying
the
relationship
between
such
factors
and
allergic
diseases
is
still
incomplete
but
is
an
area
of
rapid
expansion
and
interest
[37].
6.
Family
and
hygiene
hypothesis
With
the
use
of
skin
prick
tests
(SPT)
and
the
levels
of
specific
IgE
immunoglobins,
an
inverse
relation
is
detected
between
allergic
rhinitis
and
the
size
of
families
[38,39].
The
protective
effect
against
atopic
reactions
in
multimember
families
appears
to
be
stronger
as
the
age
of
older
children
increases
and
stronger
in
boys
than
girls
[40,41].
The
presence
of
older
children
in
the
family
decreases
the
possibility
of
allergic
rhinitis
and
eczema,
while
the
presence
of
younger
children
is
responsible
for
the
decrease
in
asthma
[42].
The
close
contact
among
children
that
is
more
likely
to
occur
in
a
multimember
family,
as
in
families
were
more
than
one
children
share
the
same
bedroom,
appears
to
offer
protection
against
the
risk
of
developing
atopic
diseases
since
the
possibility
of
exposure
to
microorganisms
or
infections
is
high
[43].
Similarly,
children
raised
with
older
siblings
or
attending
daycare
are
more
likely
to
develop
wheezing
at
the
age
of
2
years,
but
increasingly
less
likely
at
older
ages
[44].
However,
all
these
studies
are
mainly
based
on
epidemiological
data
and
cannot
easily
tackle
the
criticism
of
crudely
assessing
their
subject
without
being
able
to
prove
their
point
beyond
dispute,
and
therefore
not
sufficing
to
explain
the
effects
related
to
the
hygiene
hypothesis.
7.
Endotoxins
In
international
literature
great
interest
is
demonstrated
for
the
differences
between
civil
and
rural
populations
in
the
developing
of
allergic
diseases.
Recent
studies
have
depicted
the
fact
of
living
in
farms
and
not
generally
living
in
the
countryside
as
the
main
protective
factor
against
allergy.
For
example,
there
is
a
decrease
in
the
incidence
of
allergic
rhinitis
in
farmers’
children
in
particular,
in
comparison
to
those
of
other
rural
populations
[45,46].
A
lot
of
effort
has
been
made,
in
order
to
explain
the
immunological
background
of
this
difference,
most
of
which
is
focused
on
bacterial
endotoxins
[46,47].
Endotoxins
are
lipopo-
lysaccharides
(LPS)
that
consist
part
of
the
external
structure
of
the
cellular
wall
of
Gram-negative
bacteria.
The
immune
system
has
a
highly
sensitive
system
of
adaptors
that
detect
the
presence
of
LPS.
Endotoxins
have
the
ability
of
connecting
with
adaptor
CD14,
which
leads
to
an
increased
production
of
IL-12
through
antigen
presenting
cells,
inducing
Th
1
differentiation
that
is
not
connected
with
the
development
of
allergy
[48,49].
The
precise
mechanism
is
not
yet
totally
clear
for
as
if
the
exposure
to
endotoxins
regards
airborne
or
food
related
antigens.
In
South
Germany
and
Switzerland
[50],
it
was
proven
that
the
concentrations
of
endotoxins
in
house
dust
from
kitchens
and
mattresses
of
children
of
rural
families
are
significantly
higher
than
those
of
non-rural
families.
This
lead
to
an
even
larger
study
in
Germany,
Austria
and
Switzerland
that
presented
an
inverse
relation
between
the
level
of
endotoxins
in
domestic
dust,
and
the
developing
later
on
of
allergic
rhinitis
and
asthma.
Other
studies
that
measured
endotoxins
levels
in
non-rural
environments
[51]
with
the
use
of
skin
prick
tests
in
infants
aged
from
9
to
24
months,
presented
that
the
higher
levels
of
endotoxins
in
domestic
dust
were
associated
to
a
lower
rate
of
allergic
sensitization.
In
Estonia,
where
there
is
a
low
prevalence
of
allergy,
higher
levels
of
endotoxins
were
found
in
domestic
dust
in
comparison
to
those
in
Sweden
[52],
where
allergy
is
generally
higher.
This
fact
is
based
on
the
development
of
atopic
disease
during
the
two
first
years
of
life
only
in
children
from
Sweden.
Important
factors
that
seem
to
determine
the
outcome
of
endotoxin
exposure
in
allergic
diseases
include
the
existence
or
absence
of
exposure
in
early
life,
dose
and
frequency
of
the
endotoxins
exposure,
concurrent
exposures,
and
the
genetic
profile
that
influences
immune
responsiveness
to
endotoxins
[53].
The
issue
is
that
even
though
all
the
above-mentioned
studies
can
suggest
association,
they
cannot
prove
cause
and
effect,
leaving
space
for
rejection
to
blossom.
For
example,
one
could
point
out
that
children
with
atopic
asthma
have
their
beddings
washed
more
often
or
even
avoid
spending
time
in
barns,
thus
minimizing
exposure
and
decreasing
mattress
dust
endotoxin.
Nevertheless,
there
is
skepticism
for
the
part
that
endotoxins
seem
to
have,
not
for
their
undisputed
participation,
as
for
the
fact
that
they
might
just
be
an
easily
measurable
second
factor
of
the
same
microbial
element
[54,55].
Studies
which
combine
measuring
endotoxin
exposure
and
evaluating
genetic
background
suggest
that
allergy
and
asthma
result
from
a
complex
interaction
between
genetic
and
environmental
factors.
Although
increasing
endotoxin
exposure
is
associated
with
reduced
risk
of
allergic
sensitization
in
children
with
the
CC
genotype
at
-159
of
the
CD14
gene,
in
the
same
genotype
increased
risk
of
nonatopic
wheeze
was
demonstrated.
It
is
suggested,
that
complex
impact
of
environmental
endotoxin
may
be
enhanced
in
individuals
with
this
genotype
of
CD14
gene,
while
other
alleles
were
not
affected
[56].
8.
Hygiene
hypothesis
and
auto-immune
diseases
Many
studies
over
the
past
years
have
focused
their
research
on
the
existence
of
possible
links
between
limited
microbial
exposure
and
an
increased
rate
of
immunological
diseases,
which
have
similarly
followed
the
rise
of
allergies
during
the
last
decades.
The
deficit
of
the
immunoregulatory
mechanisms
to
terminate
inappropriate
inflammatory
responses
can
lead
to
simultaneous
upraise
in
diverse
types
of
pathology
as
depicted
in
XLAAD
syndrome
(X-linked
autoimmunity-allergic
dysregulation
syn-
drome
[57].
Apparently,
there
is
a
positive
relation
between
various
positive
markers
of
microbial
exposure
and
Th
1
regulated
inflammatory
diseases,
such
as
juvenile
diabetes
[58–60],
rheu-
matoid
arthritis
[61],
Chron’s
disease,
and
leukemia
[62–64].
On
the
other
hand,
once
again
other
studies
propose
that
a
reciprocal
relationship
between
Th1
versus
Th2
immune
develop-
ment
and
allergy,
autoimmunity
and
inflammatory
diseases
is
far
too
simplistic
when
based
only
on
the
epidemiological
data
of
the
hygiene
hypothesis.
As
a
result,
they
do
not
prove
any
kind
of
relationship
with
these
markers
of
microbial
exposure
[65–67].
9.
Antibiotics
The
possibility
of
a
relationship
between
the
use
of
antibiotics
and
the
appearance
of
asthma
or
other
atopic
reactions
is
a
modern
subject
of
interest,
even
though
it
is
hard
to
define
whether
the
reactions
are
due
to
the
infection
or
the
antibiotic
itself
[68].
Furthermore,
in
Great
Britain
any
kind
of
drug
use
before
the
age
of
two
is
related
to
double
the
possibility
of
developing
allergic
rhinitis
and
eczema,
especially
when
the
medication
contained
cephalosporins
and
macrolides
[69].
It
has
been
suggested
that
the
effect
of
antibiotics
may
be
related
to
the
effect
on
bacterial
colonization
of
the
gastrointestinal
system
at
the
early
stages
of
development
[70].
This
is
according
with
the
results
of
another
study
with
laboratory
mice,
in
which
the
antibiotic
induced
changes
in
the
intestinal
system
provoked
the
response
of
the
immune
system
to
common
allergens
in
the
lungs
[71].
Another
interesting
analysis
in
ninety-nine
centers
of
twenty-eight
countries
depicts
a
positive
relation
between
antibiotic
sales
per
capita
and
the
prevalence
of
the
symptoms
of
allergic
rhinitis,
asthma
and
eczema
[72].
E.
Prokopakis
et
al.
/
International
Journal
of
Pediatric
Otorhinolaryngology
77
(2013)
1065–1071
1068

However,
this
is
only
one
way
of
looking
at
things.
Since
in
many
countries
asthma,
particularly
at
a
young
age,
is
still
treated
with
antibiotics,
an
association
between
the
use
of
these
drugs
and
asthma
must
become
positive.
Studies
that
take
in
to
consideration
the
actual
beginning
of
asthma
and
the
timing
of
drug
use
are
rare,
and
these
studies
do
not
so
far
show
a
convincing
effect
of
antibiotic
use
on
the
development
of
asthma
and
allergic
illnesses
[73,74].
Furthermore,
it
is
demonstrated
that
the
use
of
antibiotics,
not
only
in
human
treatment,
but
also
in
the
livestock
feed,
may
significantly
increase
the
risk
of
asthma
in
children
living
on
farms
where
antibiotics
are
used
in
the
feed,
compared
to
those
where
they
are
not
used
[75].
10.
Microbial
and
virus
infections
The
best
evidence
of
the
inverse
relationship
between
exposure
to
a
particular
pathogen
and
atopy
originated
from
data
regarding
Hepatitis
A,
an
infection
associated
with
large
families
and
low
socioeconomic
level,
in
a
study
conducted
in
Italy.
In
students
of
military
academies,
it
was
found
that
the
increased
prevalence
of
specific
IgE
immunoglobins
to
airborne
allergens
was
decreased
in
half
when
there
was
evidence
of
an
earlier
HAV
infection.
These
findings
are
independent
from
age,
family
size,
birth
line,
and
area
of
residence
and
the
educational
level
of
the
parents
[76].
Two
more
studies
present
that
seropositivity
for
HAV
in
the
general
population
is
related
to
a
decrease
in
allergic
reactions
by
40%
and
37%
respectively
[77,78].
In
air
force
cadets
in
Italy
that
suffered
from
various
allergic
diseases,
significantly
lower
levels
of
antibodies
to
Toxoplasma
gondii,
Helicobacter
pylori
and
HAV
were
measured,
in
comparison
to
non
allergic
subjects
[79].
The
receptor
for
HAV
on
human
lymphocytes
is
T-cell
immunoglobulin-
and
mucin-domain-
containing
molecules
(TIM-1),
which
is
involved
in
the
regulation
of
T-cell
subsets
including
regulatory
T
cells
and
Th2
cells.
Exposure
to
HAV
might
selectively
remove
Th2
cells,
or
alter
the
balance
of
T-cell
subsets,
leading
to
the
conclusion
that
premature
exposure
to
microorganisms
through
oral
and
gastro-
intestinal
routes,
offer
protection
against
infections
of
the
respiratory
system,
and
that
the
‘‘healthy,
westernized,
sterile
nutrition’’
can
facilitate
the
appearance
of
allergic
reactions
through
the
effect
of
microorganisms
that
stimulate
the
gastroin-
testinal
lymphoid
tissue.
However,
in
some
other
studies
there
are
populations
that
do
not
present
any
relationship
between
Helicobacter
pylori
or
HAV,
and
the
developing
of
allergic
diseases
[80–82].
Furthermore,
common
childhood
infections
like
measles,
mumps
and
chick-
enpox
seem
to
be
unable
to
protect
from
allergic
disorders
[83,84],
leaving
HAV
as
possibly
the
only
resisting
stronghold
of
hygiene
hypothesis.
11.
‘‘Western’’
way
of
life
The
drastic
effect
that
modern
life
style
has
on
the
impressive
rise
of
allergic
diseases
and
the
role
that
environmental
agents
play,
are
characteristically
demonstrated
in
a
epidemiological
study
conducted
in
New
Zealand
regarding
a
population
that
was
forced
to
change
residence,
and
the
effect
that
this
change
had
to
the
frequency
of
allergic
reactions.
In
particular,
in
1966,
after
a
catastrophic
hurricane
that
stroke
the
small
island
Tokelau,
in
the
Pacific
Ocean,
New
Zealand’s
government
that
had
the
island
under
its
jurisdiction,
relocated
1950
inhabitants
to
New
Zealand.
Fourteen
years
later
the
children
that
moved
from
the
almost
primitive
conditions
to
modern
conditions
of
living,
presented
in
comparison
to
the
children
that
remained
on
the
island,
a
significantly
higher
prevalence
of
allergic
rhinitis
(28–14%)
and
asthma
(25–10%)
[85].
The
abundance
of
aseptic
products
and
practices
that
promote
a
‘virgin’
environment
for
people
in
general
and
for
young
children
in
particular,
may
have
decreased
our
exposure
to
incoming
‘danger’
signals
from
microbes
and
germs
that
has
been
part
of
our
allergic
alert
for
centuries.
One
suggestion
is
that
since
the
immune
system’s
primary
driving
force
is
the
need
to
detect
and
protect
against
danger,
and
that
it
does
not
do
the
job
alone,
but
receives
positive
and
negative
communications
from
an
extended
network
of
other
bodily
tissues,
the
lack
of
these
signals
due
to
the
‘‘clean
Western’’
way
of
life,
adds
to
the
problem
[86].
The
exact
way
that
urbanization
and
westernization
influences
allergic
diseases
is
an
ongoing
investigation
[87]–in
a
recent
study
even
having
a
tonsillectomy
before
7
years
of
age
seemed
to
increase
the
risk
of
early
onset
allergic
rhinitis
(OR
=
1.7,
95%
CI
1.2–2.5)
[88].
The
mechanism
for
the
effect
of
tonsillectomy
is
open
to
discussion,
since
possibilities
include
its
significance
as
a
marker
of
antibiotic
use
or,
alternatively,
a
marker
of
severe,
repeated,
upper
respiratory
infection
resulting
in
inflammation
and,
therefore,
increased
risk
of
sensitization
via
increased
mucosal
permeability.
However,
since
long-term
data
are
lacking,
one
can
only
speculate
that
short-term
changes
might
proceed
into
immunological
imbalance.
Regardless,
there
is
no
doubt
that
genetic
disposition
is
only
one
of
the
many
driving
forces
in
allergy
as
represented
in
the
changes
of
the
allergic
profile
from
patients
in
East
Germany
only
20
years
after
the
reunification
[89].
It
is
of
note,
that
the
increased
prevalence
of
allergic
sensitization
and
concurrent
allergic
diseases
may
be
one
of
the
factors
associated
with
the
rise
of
chronic
rhinosinusitis
[90].
12.
Vitamin
D
More
recently,
variations
in
vitamin
D
status
and
intake
have
also
been
implicated
in
allergy
development
and
considered
as
one
of
a
number
of
explanations
for
epidemiological
and
immunologi-
cal
associations
[91],
based
on
its
role
in
the
development
and
maintenance
of
lung
structure
and
function
and
in
immunity
[92,93],
even
reaching
the
point
were
one
has
started
to
ask
whether
correcting
vitamin
D
levels
affects
the
incidence
and
the
course
of
allergic
disease
[94].
Aside
from
skeletal
health
vitamin
D
regulates
the
activity
of
various
immune
cells,
including
monocytes,
dendritic
cells
(DCs),
T
and
B
lymphocytes,
as
well
as
immune
functions
of
epithelial
cells
[95].
Furthermore,
some
immune
cells
express
vitamin
D-
activating
enzymes
facilitating
local
conversion
of
inactive
vitamin
D
into
active
calcitriol
with
subsequent
paracrine
and
autocrine
effects
[96,97].
In
particular,
vitamin
D
inhibits
the
expression
of
pattern
recognition
receptors,
which
activate
innate
immune
responses
such
as
the
Toll-like
receptors
on
monocytes
and
suppresses
TLR
mediated
inflammation
[98],
decreases
immune
receptor
expression
also
on
monocyte-derived
DCs,
reduces
the
function
of
these
cells
(e.g.
chemotaxis,
antigen
presentation,
maturation)
[99,100]
and
furthermore,
induces
autophagy
in
human
macrophages,
which
possibly
contributes
to
the
defense
against
opportunistic
infections
[101].
Besides
its
effect
on
the
innate
immune
system,
vitamin
D
is
a
major
player
in
adaptive
immunity,
since
it
inhibits
T-cell
proliferation
through
decreased
Th1
cytokine
secretion
[102],
increases
IL-10
and
decreases
IL-2
production,
thereby
inducing
a
state
of
hypo-responsiveness
in
T
regulatory
cells,
similar
to
the
one
produced
by
corticosteroids
or
allergen
immunotherapy
in
anti-allergic
therapies
[103].
Therefore,
the
impact
of
vitamin
D
on
immune
functions
might
be
particularly
critical
as
prevalence
of
hypovitaminosis
D
is
high
with
up
to
30%
in
the
adult
Western
population
and
up
to
70%
in
the
elderly
or
institutionalized,
easily
awarded
the
title
of
a
‘‘pandemia’’
[104,105].
E.
Prokopakis
et
al.
/
International
Journal
of
Pediatric
Otorhinolaryngology
77
(2013)
1065–1071
1069

Even
though
the
allergy
society
anxiously
anticipates
the
moment
of
‘‘Eureka’’,
the
evidence
for
beneficial
effects
of
vitamin
D
on
allergic
diseases
is
primarily
based
on
observational
and
often
retrospective
studies,
and
while
experimental
and
pre-clinical
data
point
towards
a
protective
role
of
vitamin
D,
the
clinical
results
available
to
date
draw