Biology Reference
In-Depth Information
do with the evolutionary history of both processes. Because of the scarce
empirical research in this particular field, most analyses are supported by
hypothesis and interpretations of the theory of evolution through natural
selection. In addition, during the last decade, theoretical models of pop-
ulation genetics, genomic scans for selection and bioinformatics have
been used to explore the selective pressure of helminths and other
infectious organisms on immunity-related genes.
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e
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The following is
a brief and personal view about this interesting topic.
From an evolutionary perspective, it is theoretically plausible that
strong Th2 immune response that overcomes immunosuppression
protects against ascariasis and other helminthiases, and, in fact, there is
evidence that atopic individuals have lower intensities of helminth
infections.
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e
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A number of factors and organisms, including parasites,
have influenced the evolution of defense mechanisms of humans but,
because of the multiple phenotype similarities, the evolutionary process
of the allergic response and the immune response to helminths
are supposed to be the same, or at least very close.
21,165,166
However,
a fundamental unresolved question is if current genotypes predisposing
to allergy were selected by helminth infections or, to the contrary, those
genotypes (shaped by factors different to parasites), generating resistance
to helminths, avoided the potential lethal effects of these infections and
exerted selective pressure on them.
One of the possible consequences of the long co-evolution of
Ascaris
and different hosts is the selection, among both species, of those organ-
isms with the best genotypes to establish and maintain a parasitic rela-
tionship. Considering the complexity of these relationships, this implies
a large number of genotypes covering a variety of physiologic processes,
but, based on the latest advances in our understanding of the nature of
human
eAscaris
interactions, examples of these organisms may be, for the
parasite, those able to induce immunomodulation and for the host, those
susceptible to such immunomodulation. This condition does not presume
that immunosuppressor properties made successful parasitic helminths
totally harmless to their hosts,
167,168
but suggests that the selective pres-
sure exerted by helminths could positively select polymorphisms deter-
mining host resistance to immunomodulation and in turn this could be an
important mechanism underlying the different degrees of “resistance
phenotypes” in humans.
Among the human genotypes resulting from this bilateral adaptive
process and involving the “resistance” phenotype, two can be hypothe-
sized as described in
Figure 2.2
. Based on this model, several subpopu-
lations and outcomes are theoretically possible in regard to resistance to
Ascaris
and the effects of both the infection and parasite eradication on the
prevalence of symptoms. It could be that a group of highly allergic
subjects are insensitive to immunomodulation of any type, including that
