526 F. M. BURNET 



inactivate a certain fraction of virus, while leaving another fraction active, 

 and that will have no harmful effect on the host cells in the system. For 

 such purposes specific immune serum, used with due regard to its powers 

 and limitations, is the standard reagent. 



In the last analysis, all such uses depend on the immunological specificity 

 of viral antigens, which in some instances, and possibly in all, are of protein 

 character. Only indirect attention has so far been paid to the functional 

 significance of viral proteins, but as such studies develop it is inevitable that 

 much use will need to be made of immunological methods. In the animal 

 viruses, at least, immunological specificity has a vital part in determining 

 survival of a virus strain; it is possible that in all viruses specific protein 

 patterns, enzymatic or other, play a part in the initiation of infection. At the 

 present time, the immunological approach provides an excellent experimental 

 technique directly applicable to the finer pattern differences between pro- 

 teins and it will often provide information in both the modern technical and 

 the trivial senses beyond anything obtainable by other chemical means. 



The present contribution is concerned, not with the significance of im- 

 munity in viral disease, but with immimological procedures that have been 

 used for the characterization of viruses and for the study of the virus-host cell 

 interaction. As far as possible, the approach wiU be made as applicable to 

 plant and bacterial viruses (where immunological phenomena are of no sig- 

 nificance in nature) as to animal viruses (where they may dominate the 

 ecological situation). 



As in so many other current aspects of virology, knowledge of the anti- 

 genic structure of viruses is incomplete and patchy. Reasonably pure and 

 well-characterized protein antigens have been obtained from some of the 

 better- studied plant viruses (Schramm, 1943) and one antigen (LS) of limited 

 significance from vaccinia virus (Smadelei al., 1942). From the great majority 

 of virus types no attempt has been made to separate antigenic fractions from 

 the virus particle. 



This does not, however, detract from the value of the studies made at a 

 more superficial level. As examples may be mentioned the use that has been 

 made of appropriate antisera in analyzmg tJie relation of protein and nucleic 

 acid in plant viruses, in correlating details of morphology with function in 

 the bacterial viruses, and in demonstrating the nature of filamentous forms 

 of influenza virus. In the future we can reasonably expect that elaboration of 

 work on the soluble complement-fixing antigens of animal viruses will throw 

 much light on the process by which virus-specific protein is synthesized in the 

 infected host cell. 



There is still some question as to the nature of the process by which an 

 animal becomes immune to a virus disease after clinical or subclinical infec- 

 tion. Many would consider that cellular immunity of some type must be 



