<P> The body's first line of defence against viruses is the innate immune system . This comprises cells and other mechanisms that defend the host from infection in a non-specific manner . This means that the cells of the innate system recognise, and respond to, pathogens in a generic way, but, unlike the adaptive immune system, it does not confer long - lasting or protective immunity to the host . </P> <P> RNA interference is an important innate defence against viruses . Many viruses have a replication strategy that involves double - stranded RNA (dsRNA). When such a virus infects a cell, it releases its RNA molecule or molecules, which immediately bind to a protein complex called a dicer that cuts the RNA into smaller pieces . A biochemical pathway--the RISC complex--is activated, which ensures cell survival by degrading the viral mRNA . Rotaviruses have evolved to avoid this defence mechanism by not uncoating fully inside the cell, and releasing newly produced mRNA through pores in the particle's inner capsid . Their genomic dsRNA remains protected inside the core of the virion . </P> <P> When the adaptive immune system of a vertebrate encounters a virus, it produces specific antibodies that bind to the virus and often render it non-infectious . This is called humoral immunity . Two types of antibodies are important . The first, called IgM, is highly effective at neutralising viruses but is produced by the cells of the immune system only for a few weeks . The second, called IgG, is produced indefinitely . The presence of IgM in the blood of the host is used to test for acute infection, whereas IgG indicates an infection sometime in the past . IgG antibody is measured when tests for immunity are carried out . </P> <P> Antibodies can continue to be an effective defence mechanism even after viruses have managed to gain entry to the host cell . A protein that is in cells, called TRIM21, can attach to the antibodies on the surface of the virus particle . This primes the subsequent destruction of the virus by the enzymes of the cell's proteosome system . </P>

When does the genetic material of a host and a virus combine