IMMUNOLOGICAL METHODS IN THE STUDY OF VIRUSES 539 



TABLE I 



The Number of Mobphological Particles Needed to 

 Give One IDgo for Some Representative Viruses " 



Virus Number of particles 



Vaccinia 366; 4.2 



1.0^ 



Psittacosis group 43; 100 



Myxovirus group — 



Influenza A and B 10 



Newcastle disease 5 



Mumps 100 



Poliovirus 20,000 



35, 60, 120" 



Western equine enceplialitis virus 14** 



" Isaacs, 1957. ^ Overman and Tamm, 1956. 



« Schwerdt and Fogh, 1957. <* Dulbecco et al., 1956. 



Similarly, it is found that a given virus-serum mixture that is neutraUzed 

 for a moderately susceptible host may be highly infective for a more suscept- 

 ible species. Working with the same strains of influenza virus at different 

 stages of adaptation to growi;h in the allantoic cavity, Burnet (1943) foimd 

 that neutrahzation by homologous immune serum was more active against 

 the less completely adapted substrain. Von Magnus (1951) showed that 

 antibody against Theiler's virus could not be demonstrated by intracerebral 

 incculation in adult mice, but with baby mice from a virus-free colony intra- 

 peritoneal inoculation gave clear-cut positive results. 



1. Neutralization as Tested in the Experimental Animal 



The crude facts are simple. If infective material is mixed with immune 

 serum and inoculated by an appropriate route mto a susceptible host, its 

 infectivity will be less than if normal serum is used. Closer study, however, 

 raises difficulties of interpretation in every direction. When the process by 

 which vaccinia virus is neutrahzed first came under study around 1928-1935 

 it soon became evident that serum had no directly destructive action on the 

 virus. A mixture judged to be fuUy neutralized by failure to give a lesion in 

 intradermal inoculation in the rabbit could be reactivated by simple dilution 

 (Andrewes, 1929, 1930). Andrewes also commented on the great variability 

 in the results of inoculating the same serum-virus mixtures into different 

 individual rabbits. Evidence that union between virus and neutralizing anti- 

 body took place in vitro was eventually obtained by Smith (1930); in 1937 

 Salaman showed that protective antibody could be absorbed with elementary 



