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normal immune functions in practically all animal models as well as in many wildlife spe-
cies (Krzystyniak et al. 1995; Luebke et al. 1997; Voccia et al. 1999; Galloway and Handy 2003).
The immune system is quite well conserved through animal taxa and is sensitive to
environmental stress. During evolution, the immune system has developed its effector
mechanisms, through host-infectious agent interactions, thereby protecting the host
against potential pathogens.
By definition, immunological biomarkers or immunomarkers are biomarkers that result
from alterations to the immune system caused by exposure to one or more xenobiotics.
Immunotoxic effects may occur either (1) by immunosuppression, which is a decline in host
resistance toward a large spectrum of infectious agents, as well as the increased incidence
of cancers, or (2) by immunostimulation, whose consequences are more complex and usu-
ally characterized by a qualitatively abnormal response such as hypersensitivity reactions
like allergies or asthma, flulike reactions, and the occurrence of autoimmune diseases for
the acquired immune component at the innate level, increased phagocytosis and mediators
of inflammation could be observed as well (Luster et al. 1992a, 1992b, 1993; Revillard 1994;
Kouassi et al. 2003). The outstanding value of the immune system relies on its high sensitiv-
ity, intimately linked to its sophisticated structure and integration of its function.
6.2 Case of Invertebrates—Bivalves
6.2.1 Introduction
A growing body of data on immunological biomarkers comes from experiments using
bivalves as animal models to study the fundamental effects of toxins on the innate
immune level. Bivalves, being filter-feeding organisms, are able to accumulate in their
tissues xenobiotics at concentrations considerably higher than those found in the ambi-
ent waters (Phillips 1995). They also represent an experimental model of choice for both
in vitro
and
in vivo
approaches with organisms exposed in aquaria, caged animals, and
indigenous specimens.
Invertebrates, like all other organisms, are permanently exposed to antigenic stimula-
tion. Whereas in vertebrates the immune system includes acquired or adaptive immunity
and innate or natural immunity, in invertebrates protection of the organism is assured
solely by innate or natural immunity (Mydlarz
et al. 2006). However, a primitive immuno-
globulin (Ig) system seems to exist in invertebrates as indicated by the presence of a mol-
luskan defense molecule (MDM) (Hoek et al. 1996). MDM is preferentially expressed in
hemocytes and in granular cells. It is released in the hemolymph as a cytokine for nonself
recognition and is involved in the agglutination and opsonization of foreign micropar-
ticles in the snail. However, the role of Ig in immune systems is not well understood at
present, and more research will be needed to understand these fundamental aspects of
immunological functions in invertebrates. Allograft inflammatory factor-1 has also been
found in sponges and abalones; this is a cytokine involved in the recognition and in the
rejection of allografts (Müller and Müller 2003; De Zoysa et al. 2010). In bivalve mollusks,
natural immunity is composed of cellular components (mostly hemocytes) and humoral
components consisting of molecules circulating in the hemolymph. Hemocytes, distrib-
uted in all tissues, represent the main immune cell population. No site of hemopoiesis has
been described yet in this group, but observation of some cytological diversity suggests
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