Biology Reference
In-Depth Information
specific immunologic effects to ascariasis alone even when blood cells are
stimulated in vitro with Ascaris antigens because of the high degree of
immunological cross-reactivity between STH antigens.
50
The existence of protective immunity to ascariasis in humans remains
controversial but is suggested by the observation that, under conditions of
high levels of transmission, the prevalence and intensity of A. lumbricoides
declines with age.
51
Non-immunological factors, such as changes in
behavior with age, are reasonable alternative explanations for this
observation. Even so, a number of investigators have tried to identify
immunological parameters associated with protection.
A study of reinfection following treatment was unable to attribute
a protective role of specific antibodies to adult and larval stages of
A. lumbricoides against current infection or reinfection,
35
while other
studies have suggested that specific IgE might have a protective role.
52,53
A randomized clinical trial giving anti-IgE for the treatment for atopic
diseases, which reduced circulating levels of IgE to negligible levels, did
not show a significant effect of anti-IgE treatment on susceptibility to
infection with STH parasites, although a trend of increased risk of infec-
tion (mainly ascariasis) was seen among asthmatic patients receiving anti-
IgE compared to placebo.
54
It seems reasonable to suggest, therefore, that IgE, particularly specific
IgE, has a role in protective immunity to A. lumbricoides infection either in
the initiation of allergic-type inflammatory responses against the parasite
or in the amplification of other Th2-mediated mechanisms. Individual
Th2 effector mechanisms could be important in mediating protection
against different life-cycle stages of A. lumbricoides: IgE-mediated hyper-
sensitivity in the intestine may be important in expulsion of juvenile and
adult parasites while other Th2 effector mechanisms, which may or may
not include IgE, could be important in the killing of parasite larvae in the
liver and lungs.
55
A study conducted in a hyperendemic community for ascariasis
showed that the age-dependent decline in prevalence of infection was
mirrored by an increase in Th2 cytokine production (IL-4, IL-5, IL-9,
IL-10, and IL-13) by antigen-stimulated PBLs from older individuals,
56
and attributed the age-dependent decline in infection to an increase in
Th2 cytokines with age. An infection
reinfection study of individuals
infected with A. lumbricoides and T. tr ichiura showed that the elevated
production of Th2 cytokines (IL-5 and IL-13) by PBLs stimulated
with STH antigens was associated with resistance to reinfection.
57
An alternative approach has been to identify individuals that are
resistant to infection (i.e. that are free of infection despite residence in
a highly endemic community), so-called putative immunes (PIs), and
compare immune parameters with infected individuals from the
same community. Such a study in Nigeria observed that resistance
to infection was
e
associated with increased levels of
innate
