94 WALLACE 



reducing infectivity of plant viruses. Non-specific reduction takes place 

 immediately upon mixing normal antiserum and antigen. 



B. Each antiserum has a specific effect in reducing the infectivity 

 of the virus used in making that antiserum. Specific effect increases 

 with time. 



C. Specific reduction in infectivity results from the action of anti- 

 serum on the virus and not on the host. 



D. The virus is neutralized rather than inactivated since some in- 

 fectivity is regained when neutralized mixtures of virus and antiserum 

 are either diluted or incubated with pepsin. 



Serological reactions as an aid in identification of viruses — The 

 specificity of serological reactions permits their use in the identification 

 of viruses. However, inasmuch as these reactions are only group specific, 

 identification by this means is not exact. Different strains of the same 

 virus usually react similarly when tested against one another in straight 

 serological tests. Reaction between the virus of one strain against the 

 antisera of another strain, however, does not prove that these strains are 

 identical antigenically, but shows merely that the two strains contain 

 certain common antigens. On the basis of cross-absorption procedures, 

 serologic tests with certain related virus strains suggest that each virus 

 is not a simple unit antigen but that each may carry a number of dif- 

 ferent determinant groups which will likewise be contained in the 

 antisera of each virus. 



It has been observed that the range of antigen dilution over which 

 precipitation occurs may vary widely when two related virus strains 

 are tested against a given antiserum. 



The lack of sufficient antigen in expressed sap is no doubt sometimes 

 responsible for failures to obtain precipitin reactions. In fact, there are 

 recorded instances where concentration by precipitation or by high- 

 speed centrifugation have established strain relationships that were not 

 detected when crude sap was used. 



With an understanding of the phenomena listed above and their 

 use in serological technique, it seems that these methods may be not 

 only of value in grouping virus strains but that they may also become 

 useful in indicating the degree of relationship between different strains. 

 From the standpoint of virus classification it appears that viruses must 

 be arranged in larger groupings or families on the basis of properties 

 or characteristics other than serological responses, but the fact that 



