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damaging or traversing epithelial layers, or indirectly, by inducing tissue-
damaging infl ammatory responses. If microbial pathogens invade the
host, innate and adaptive defence mechanisms are activated for preventing
further spread of the infection (Gómez and Balcázar, 2008).
The fi sh immune system possesses two integral components: (i) the
innate, natural or non-specifi c defence formed by a series of cellular and
humoral components, and (ii) the adaptive, acquired or specifi c immune
system characterized by the humoral immune response through the
production of antibodies and by the cellular immune response mediated
by T-lymphocytes.
However, it is believed that marine fi sh larvae do not have the ability
to develop adaptive immunity during the early stages of development.
In this regard, the innate immune system is probably the major defence
against microbes (Vadstein, 1997). These parameters include phagocytes,
various lectins, lytic enzymes, antibacterial peptides and proteinase
inhibitors (Magnadottir et al., 2005). A variety of substances could, at
least theoretically, activate these innate parameters. These substances or
immunostimulants have recently been defi ned as “a naturally occurring
compound that modulates the immune system by increasing the host's
resistance against diseases that in most circumstances are caused by
pathogens” (Bricknell and Dalmo, 2005). In general, these are compounds
of bacterial, fungal or plant origin, especially composed of polysaccharides,
which activate the pattern recognition receptors/proteins of the immune
system resulting in a varied immune response.
Previous studies have demonstrated that administration of
immunostimulants to fi sh is benefi cial, providing improved protection
against bacterial and, to a lesser extent, viral infections (Vadstein, 1997;
Bricknell and Dalmo, 2005). Thus, the immunomodulation of larval fi sh
has been proposed as a potential method for improving larval survival
by increasing the innate responses until its adaptive immune response is
suffi ciently developed to generate an effective response to the pathogen.
Lipopolysaccharides (LPS) are one of the immunostimulants widely
used in experimental fi sh larval aquaculture, as they can trigger various
immune components. Sea bream ( Sparus aurata ) larvae when treated
with LPS exhibited signifi cantly high anti-protease activity, lysozyme
activity and total globulin level, and also conferred protection against
Photobacterium damsela (Hanif et al., 2005). Similarly, Magnadottir et al.
(2006) reported enhanced survival by immunizing Atlantic cod ( Gadus
morhua ) with LPS.
The range of potential immunostimulants for larval fi sh is quite limited
and a short review can be found in Table 1.2. Although these studies have
demonstrated some benefi cial effects of immunostimulants in terms of
survival and growth, the relative importance of the immune mechanisms
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