The
immune system
The
immune system plays an important role in homeostasis by maintaining animal
health in both innate and adaptive ways (Rice & Arkoosh 2002). Innate
mechanisms are foundinagnathan and gnathostome fishes, and consist of immune
factors that block invasion by potential pathogens. For example, the external
layer of skin and scales is a physical barrier to infectious organisms. In
addition, the sticky, viscous consistency of the mucus secreted by fish
epithelial cells probably helps to trap microorganisms, and the mucus can
contain antibodies and chemicals that destroy or inhibit bacteria (Bernstein et
al. 1997). The volume of mucus secreted may increase in stressful situations,
indicating a response on the fish’s part to shield itself from potentially
harmful chemicals, microorganisms, or other agents. Other parts of the innate
response include inducible phagocytic cells that can attack and destroy
potential pathogens, cytotoxic cells that destroy cells infected by viruses,
and the complement system of proteins that attack the membrane of invading
cells (Rice & Arkoosh 2002).
The
adaptive response, in contrast, involves the detection of an invader and the
creation of specialized responsemechanisms to identify and destroy it. This
response has not been seen in agnathans, but is present in the
gnathostomes(Bernstein et al. 1997). The organs primarily responsible for this
response are the kidney, thymus, spleen, and gut.
The
adaptive response includes both cellular and humoral components
(Rice & Arkoosh 2002). The cellularcomponent of the adaptive response
includes cytotoxic Tells that can destroy cells that have become infected
by viruses or that show signs of becoming cancerous. The humoral response
involves the detection of specific invading compounds (antigens) and the
production of antibodies designed to bind to them. These antibodies tag the
antigenic particles for destruction by other components of the immune system,
such as macrophages that engulf and digest the tagged antigens, or complement
proteins that destroy tagged cells by puncturing their membranes. Antibody
structure of the Chondrichthyes is somewhat similar to that of the higher bony
fishes and mammals. The structures of the genes responsible for antibodies are
quite different, however, with those of the bony fishes somewhat intermediate
between those of the Chondrichthyes and those of mammals (Bernstein et al.
1997).
In
mounting an antibody response, the immune system also produces memory cells that
remain in the blood stream for extended periods (Rice & Arkoosh 2002).
Memory cells help the animal’s immune system react quickly if it encounters the
same antigen in the future. Consequently, subsequent exposures to an antigen
are dealt with quickly and the antigens are destroyed much more quickly than
waste case during the initial exposure to that same antigen. Vaccinations,
which have become important in fish culture, take advantage of memory cell
development. By exposing fish to a less virulent form of a pathogen, the fish’s
immune system can defeat this initial infection and will retain memory cells to
help it respond quickly and more effectively to subsequent exposures to a
potentially more virulent form of the pathogen.
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