Biology Reference
In-Depth Information
stronger than any described immunomodulator phenomenon accompa-
nying bacterial or viral infections excepting human immunodeficiency
virus (HIV) infection. This has reinforced the idea that parasite infections
have played a major role in controlling the allergic responses; and the lack
of this control, because of an improvement in hygiene conditions, may be
a relevant factor in allergy prevalence.
154
Therefore, the high prevalence
of asthma that is observed in some urbanized zones of the tropics, where
helminth infections, such as ascariasis, are still present but with less
intensity than in the past, may be explained, among other factors, as
a consequence of the particular historical moment of the ancient and
complex relationships between parasites and the immune system: a point
where, because of several reasons, the immunostimulator effects of
helminths on the Th2/IgE responses predominate. As mentioned, the
type and distribution of parasites, as well as the frequency, intensity, and
immunomodulator effects of helminth infections, are not the same
throughout the world. In the tropics, both the immunosuppressive and
the Th2-immunopotentiating effects can be detected, the latter being more
frequent at the population level. Therefore, in this changing world, three
distinct relationships between helminths and the human immune system
can be recognized: one with chronic, heavy intensity infections and
mainly immunosuppressive; others of intermittent low parasite-load
infections, predominantly IgE-enhancer and associated with urbaniza-
tion; and a third, with absence of
infections, where there is no
parasite-derived immunoregulation.
The possible consequences of each of these conditions on the
development of allergic diseases have been already analyzed, but three
additional comments are pertinent. First, allergic diseases are highly
dependent on the effects of other environmental factors; for example, the
potential effects of changes associated with urbanization, differences in
diet and lifestyle, physical activity and housing
155
should also be
considered. Second, a comprehensive study of the human
e
helminth
relationships should include the genetic and evolutionary components,
which may provide relevant information about the current host
e
parasite
interactions.
156
e
158
Third, since the pathogenesis of ascariasis and other
helminth infections, as well as that of asthma, are highly influenced by
genetic factors, these will limit the proportions of individuals that
establish any of the proposed relationships in a given population.
49
ALLERGY AS A PROTECTIVE FACTOR FOR
ASCARIA
SIS: THE OTHER FACE OF CO-EV
OLUTION
Although this chapter is mainly about the effects of ascariasis on
allergy, some words about the contrary are necessary as they have to
