The natural immune system consists of preexisting or rapidly inducible effector components as a first line of defense against pathogens to which the host has not previously been exposed. Such a system is necessary as the B and T cell specific responses to any pathogen require the selective expansion of high-affinity antigen-specific clones which take several days to develop. An unimpeded replication of a virus producing 10⁵ progeny per cell could potentially infect all of the cells in the host after only three or four replication cycles. Without the natural immune system holding infections in check, pathogens could overwhelm the host before the T and B cell responses become effective.