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concentrations are usually encountered in the blubber (Severinsen et al. 2000). General bio-
logical traits of the individual, particularly age and gender, also affect the tissue levels of
pollutants (Krahn et al. 2009). Contaminant burdens may differ between age class, partly
as a result of historical changes in environmental contamination and the long life span of
most marine mammals (Honda and Tatsukawa 1983). Whereas persistent pollutant concen-
trations increase in males during their lifetime, females transfer these substances to their
offspring during each pregnancy and lactation (Wagemann et al. 1988; Tanabe et al. 1994;
Borrell et al. 1995). The intensive offloading of contaminants via gestation and lactation
constitutes a major elimination and detoxification mechanism of lipophilic contaminants
stored in the fatty tissue of females and is also a significant route of exposure for suckling
pups or calves at a moment in their development when they could be more susceptible to
the immunotoxic effects of these substances (Beckmen et al. 2003; Pillet et al. 2005).
Metabolism or biotransformation of pollutants within marine mammals can also affect
pollutant concentration patterns. Interestingly, the by-products are often highly reactive
intermediates, and have been shown to elicit toxic effects in marine mammals. For exam-
ple, DBT, a metabolite of TBT, exhibits much stronger immunological toxicity to harbor
seals
Phoca vitulina
than the parent compound (Frouin et al. 2008).
6.3.2 Immunotoxicity of PTSs in Marine Mammals
The demonstrated exposure of marine mammals to PTSs leads to the question of the poten-
tial immunotoxic effects and the consequences on the health of marine mammal popula-
tions. The possible contribution of exposure to immunotoxic environmental pollutants
became a major concern in late 1980s and early 1990s when a severe epizootic resulted in
massive lethality in harbor (
Phoca vitulina
) and gray seal (
Halichoerus grypus
) populations
inhabiting a relatively polluted area in northern Europe. In Canada, the high prevalence of
cancers in the beluga whale
Delphinapterus leucas
population of the St. Lawrence Estuary
(Quebec, Canada) concurs with the impact of anthropogenic chemicals on the immune
system of this heavily contaminated population.
A great deal of attention was therefore accorded to assess the risk of immunotoxic effects
resulting from exposure to environmental contaminants in marine mammals, especially
on populations heavily exposed to anthropogenic chemicals. Consequently, the effort
to apply the recommended tier approach promoted the development of immunological
tools needed to address the immune response of marine mammals. It turns out that the
immune system of marine mammals appears very similar to that of terrestrial mammals.
NK cell cytotoxic activity, phagocytosis, and inflammatory cytokines have been demon-
strated in pinnipeds and cetaceans, revealing the existence of an innate immune response
(De Guise
et al. 1995a; Ross et al. 1996; De Guise et al. 1997b; Pillet et al. 2000; Noda et
al. 2003). The innate immune response represents the host's first line of defense against
pathogens and is essential to the initiation of the adaptive immune response. In contrast to
the innate immune response, the adaptive response is specific to the pathogen and conse-
quently more efficient. The adaptive immune response involves humoral and cell-mediated
immunity, and both have been identified in several species of marine mammals. The B
lymphocytes are the main effectors of the humoral immune response and their antigen-
recognition molecules are Ig. Membrane-bound Ig on the B-cell surface serves as the cell's
receptor for antigen. The Ig of the same antigen specificity is secreted as an antibody (Ab)
by terminally differentiated B cells after the binding of the antigen to the Ig at the cell's
surface. Although gross similarities have been shown to exist in the structures of the dif-
ferent Ig classes, species-specific differences in binding affinities require the development
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