540 F. M. BURNET 



bodies. Even as late as 1935, however, Sabin (1935) considered that the 

 evidence pointed strongly toward an effect of antiserum on the cell rather 

 than on the virus. 



This early work was sufficient to show, first, the complications associated 

 with using the intact animal, in this case the rabbit, as the test object, 

 including difference in residts according to the route of inoculation, irregu- 

 larity in replicate tests plus the possible comphcation of early active im- 

 munity and, second, the strong indication that a proportion of active virus 

 could persist m the presence of a large excess of antibody with, as a virtual 

 implication of this, the possibihty of reactivation on dilution. 



2. Neutralization on the Chorioallantois 



With the development of pock-counting methods on the chorioallantois, 

 it seemed possible that most of these disabilities could be overcome and 

 Burnet and associates (1937) carried out a large volume of work on the 

 neutrahzation of most of the viruses then known to produce discrete foci on 

 the chorioallantois. This led to the development of a general interpretation 

 which was summarized at the time as follows: 



"(i) Virus inactivation by immmie serum results primarily from union of 

 antibody to the virus surface. This union is a reversible one; it takes place 

 at a rate and reaches an equilibrium determined by the ordinary laws of 

 reversible chemical unions. 



(ii) Union has no intrinsic inactivating effect on the virus: the hiactivation 

 is the result of the interaction between susceptible cell and antibody-coated 

 virus particle. Certain susceptible cells are protected against infection by 

 lesser degrees of antibody-coating on the virus particle than are required by 

 other types of cell. 



(iii) In practice the experimental results obtained with any given virus 

 will deviate from the theoretical values owing to the operation of one or more 

 of the following factors: 



(a) variations in susceptibility within the virus popidation, 



(b) the time needed to reach equihbrium, 



(c) secondary events between time of inoculation and mitiation of lesion, 



(d) secondary changes mcluding aggregation and irreversible antibody 

 union, 



(e) local or general passive immunity produced by antibody inoculated 

 with the virus." 



3. Application of Plaque Techniques 



The apphcation of plaque techniques to tissue culture sheets infected with 

 Western equine encephalomyehtis (WEE), pohovims and other viruses by 



