The increasing incidence of atopic dermatitis and other allergic diseases
in the last 30 years, especially in industrialized countries, has led
to some serious thinking about the likely causes. Atopic dermatitis patients
exhibit increased susceptibility to bacterial, viral and fungal microorganisms
because of weakened immune systems. These microorganisms play an important
part in the initiation and/or aggravation of AD.
To explain the increasing incidence, environmental factors like air pollution,
poorly ventilated homes and diet have been cited. However, consistent
evidence in support of this view is lacking.
A new ‘hygiene hypothesis’ was put forward recently. According
to this hypothesis, increasing susceptibility of people to atopic dermatitis
is the result of decreased exposure to microorganisms, and thereby to
infections, early in life.
The theory is based on certain irrefutable facts. The changed modern
life style with its use of antibiotics and much increased hygienic and
living standards have created a highly sanitized world in which the children
are being brought up. As a result, children are being deprived of the
much-needed exposure to microorganisms which play such a vital role in
increasing their natural resistance and immunity to infections in later
life. Also, desire for higher living standards has led to smaller families
where frequency of atopic dermatitis is higher due to the fact that children
in small families are less exposed to microorganisms. In larger families,
children in the company of other children are more exposed to microbes
and therefore they develop a higher tolerance to the development of eczema.
But the theory could not explain why some parasitic worm infections,
which induce high levels of immunoglobulin E in the blood, do not cause
allergy. Moreover, no such microorganisms have been discovered. As many
as 64 studies published between 1966 and 2004 failed to cite any conclusive
evidence that tuberculosis bacteria reduced the risk of eczema. On the
contrary, some childhood bacterial and viral infections were found to
increase the chances of developing eczema. However, microorganisms in
a farming environment were found to reduce the risk of eczema development.
This led to the view that the development of atopic dermatitis depended
partly on the nature and the degree of exposure rather than infection
There are mainly two types of skin colonizing microorganisms, which have
a major role in the development of eczema. These are staphylococcus bacteria
and malassezia furfur.
Staphylococcus bacteria: In eczema, bacterial colonization with staphylococcus
bacteria is most common, found in almost 90% patients. The severity of
atopic dermatitis increases with the bacterial count and a decrease in
the count results in improvement in the condition. The factors involved
in changed skin colonization of staphylococcus bacteria are, changed skin
barrier, increased bacterial adhesion, defective bacterial clearance and
reduced immune response. Further, the dry and cracked skin may facilitate
The immune system of the skin is the first line of defense against microorganisms.
When the skin is damaged various kinds of antimicrobial peptides are produced
which are quite effective against gram-negative microorganisms. However,
these peptides are not as effective against the gram-positive staphylococcus
bacteria. The immune system in atopic dermatitis is further weakened as
the production of certain beneficial peptides in the sweat glands is reduced.
These peptides, in healthy people, are quite effective against some pathogenic
microorganisms but not so in atopic dermatitis affected patients. In atopic
dermatitis affected people, even sweat production is reduced.
Malassezia furfur: The yeast, malassezia microorganism, is commonly part
of the flora found on human skins and is plentiful in sites which produce
sebum, such as the scalp, chest, and back. It is believed to contribute
to atopic dermatitis initiation. These yeasts have been implicated in
the pathogenesis of atopic dermatitis by inducing the production of IgE
antibodies. Healthy people have developed IgG antibodies to these microorganisms,
but in 30% to 80% of atopic dermatitis patients there is IgE and/or T-cell
reactivity to the organism. Patients with atopic dermatitis on the head
and the neck regions are more likely to produce malassezia specific IgE
antibodies and a higher level of skin colonization, than patients with
atopic dermatitis located elsewhere on the body. The malassezia microorganism
is rarely produced when the skin is unaffected. This, combined with the
fact of high prevalence of Type I sensitivity indicates that they have
an important role in atopic dermatitis development. Various tests have
shown malassezia present in 70% patients.
Malassezia is also active in promoting a variety of inflammatory responses,
such as, by stimulating keratinocytes. However, antifungal studies have
not been able to find the clinical significance of malassezia induced
allergy in eczema.
In future, manipulation of the natural immune response to microorganisms
may open the door to novel treatment methods. For example, a preliminary
study on administration of probiotic bacteria to atopic dermatitis patients
showed a beneficial effect on them.