Abstract
When a mixture of virus and antiserum is titrated in animals it is the usual experience to find that the infectivity of the virus has been reduced but not abolished. By convention, one refers to such a mixture as representing so many units of neutralized virus, and so many units of active virus. In some instances this convention has been literally interpreted to reach conclusions probably not justified by the facts. In particular, decisions have been made regarding the reversibility of neutralization, and the heterogeneity of the viral population, on the insecure assumption that “neutral” and “active” viral particles in the mixture are different. It is not our purpose to question these conclusions, but to inquire into the validity of the underlying assumption.
The experiment described in Table I serves as an illustration of some of the difficulties inherent in an all-or-none hypothesis of neutralization. When the neutralization effected by a given exposure of bacteriophage to antiserum is evaluated by enumeration on three different media, 3 different results are obtained. With unsensitized phage, the media consistently fall into a definite order of effectiveness, but this order is profoundly changed after the phage has been exposed to antibody. Thus the medium representing the highest degree of “host-resistance” 1 to untreated phage, exhibits no unusual resistance after the phage has been sensitized. This experiment can be modified in many ways, but in each case it is evident that the active sensitized phage is qualitatively different from the unsensitized phage. Similar observations with the animal viruses 2 might be interpreted in the same way. The advantage of experiments with the simpler system lies in their susceptibility to further analysis, especially because the effects of uncombined antibody can be excluded.
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