| Summary: | Natural resistance is now coming to be recognized as a potentially important phenomenon in host defense against infection and ma lignancy. Genetically controlled resistance mechanisms are usUally effective early in infection and before conventional immune responses are generated. Comparisons of experimental systems where natural resistance plays a prominent role demon strate the complexities of the host defense mechanisms involved, as evidenced in the present volume. Nevertheless, some com mon components of genetic resistance are discernible and largely comprise natural killer cells, macrophages, and interferon These and additional factors would seem to constitute a first line of de fense in host resistance against both viruses and tumors. It is evi dent that considerable variation in the relative importance of di stinct mechanisms may be found among various resistance sy stems and that, most likely, additional effector functions will be discovered. Resistance to tumors and most viruses is under polygenic control, has a complex mode of inheritance, and depends on appro priately complex effector mechanisms. Instances, however, whe re a single gene locus determines resistance or susceptibility to a virus, as in the case of resistance to flaviviruses or influenza viru ses, would seem to offer good prospects for elucidating the basic factors involved. Resistance to influenza virus would indeed seem to represent a comparatively simple situation: resistance is expressed at the host cell level, and interferon is its main media tor. The present volume provides insight into current concepts of such resistance mechanisms
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