In order to combat the viral infection, the host -cells synthesize special coded proteins called interferons .These by intact animal or cultured cells in response to the viral infection ,and believed to be body's first line of defense against viruses .In fact,these inhibit replication of viruses.Various interferons, known so far ,fall under three main categories [1]IFNa,[2]IFNb,[3]IFNy .All vertebrates species are able to synthesize interferons . However ,normal cells do not generally synthesize interferons until these are induced to do so .Their synthesis is induced by the viral infection .The different kinds of the host-cells .The IFN -a , is produced predominantly by leucocytes ,IFN-B mostly by fibroblasts and IFN-y only by lymphocytes . Antiviral Activity of Interferons Interferons were first recognized to interfere with the viral infwection in cultured cells .In intact animals these are produced soon after the virus infection , leading to decrease in the virus -production .Apparently ,these act as antibodies against viruses .In fact, these induce synthesis of other proteins which inhibit the viral replication . NATURE OF VIRUSES Since the discovery of viruses , the controversy has been going on whether they are living or non- living objects because they have some of the characteristics of the both .Many biologists consider them non-living because of their following characteristics : 1. Viruses are not in the form of cells.They lack cell membrane and cytoplasm . 2. They are composed of aggregations of the units of heredity enclosed in a protein sheath or coat . 3. They do not ingest food . 4 .They do not have energy producing structure nor they respire like cellular life . 5. They can be crystallised like an ordinary substance and stored for long periods in bottle .
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