Page 1454 - Veterinary Immunology, 10th Edition

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FIG. 43.3 The mechanism of RNA interference, an important
defensive mechanism in invertebrates (and plants). Double-
stranded RNA should not be present in the cytoplasm of normal
healthy cells. Its presence indicates that an RNA virus is infecting a
cell. This dsRNA is degraded by an enzyme called dicer into short
fragments (short-interfering RNA). The short fragments are then
stabilized by a set of proteins called the RISC complex. Half of
these siRNA fragments will be complementary to viral messenger
RNAs within the cell. As a result, they will bind specifically to the
mRNA. Once this happens the mRNA will be degraded.

Adaptive Immunity

Proteins belonging to the immunoglobulin superfamily have been

detected in arthropods, echinoderms, mollusks, and
protochordates. In insects, for example, a member of the
immunoglobulin superfamily called Dscam can be diversified by
alternate splicing. Isoforms of Dscam are expressed in immune

tissues and secreted as soluble proteins into the hemolymph.
Drosophila species have the potential to express more than 18,000
isoforms of Dscam. Individual hemocytes may express 14 to 50

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