Biology Reference
In-Depth Information
that patients predisposed to asthma, with a strong pro-Th2 genetic
background, infected at an early age with reinfections and permanently
exposed to mite allergens probably have a stronger IgE response to
allergens and more severe clinical symptoms. It is also important to
consider that cross-reactivity between
Ascaris
and other nematodes, such
as
Ancylostoma duodenale
,
Strongyloides stercoralis
,
Trichuris trichiura
,
Necator americanus
, and
A. simplex
has been described.
120
e
124
In addition
to confounding the serologic diagnosis of helminthiases, cross-reactivity
could play a role in the pathogenesis of allergic diseases in the tropics,
especially during helminth
e
helminth co-infections.
The Role of ABA-1 (Asc s 1) and Tropomyosin (Asc l 3)
in the Ascariasis/Allergy Relationship
Although both allergens are able to induce IgE response, there are data
suggesting that ABA-1 and Asc l 3 have different capacities to induce
protective antibody immunity or allergy sensitization. In humans, the IgE
and IgG responses to ABA-1 have been more related to protection
32
than
to allergic symptoms; in contrast, tropomyosin is a well-recognized
invertebrate pan-allergen and Asc l 3 has a high degree of homology
and cross-reactivity with mite tropomyosins. However, the possibility
that ABA-1 may induce allergic symptoms has not been ruled out. In
addition, IgE response to tropomyosin has been associated with resistance
to helminths.
125
Further studies are therefore necessary to define the
particular impact of each of these allergens. In a case
e
control survey,
evaluating the relationship between IgE,
Ascaris
extract, and asthma, we
found a statistically significant association that disappeared when
adjusting for mite-specific IgE, which confirms the importance of mites as
triggers of asthma in the tropics. This weak association could be due to
mite
eAscaris
cross-reactivity because no association was found with the
IgE antibodies to ABA-1, which has no cross-reactivity with mite allergens
and seems to have low allergy-inducer potential.
Among the mite
eAscaris
cross-reactive allergens,
Ascaris
tropomyosin
is a good candidate. In the same study, the frequency of sensitization to
rAsc l 3 was greater in asthmatics than in controls. Although adjusting for
covariates such as specific IgE to mites lowered the significance to
borderline, when analyzed as a continuous variable, specific IgE levels to
rAsc l 3 were significantly higher in asthmatic patients than in controls.
49
These findings, although not defining an independent association of Asc
l 3 with asthma, suggest that it may be a risk factor for asthma symptoms
in the tropics, especially to individuals with the predisposition to recog-
nize Asc l 3 cross-reactive epitopes. The C-terminal end of this allergen has
high sequence similarity with IgE binding epitopes found in tropomyosins
from mite, cockroach and other invertebrates. The actual role of the IgE
