134 THE PHYSICS OF VIRUSES 



some importance, a short description of the major findings in this 

 subject will be given. 



Neutralization of Infectivity 



If antibody is incubated for a standard (long) time with active 

 virus, and the percent viable virus is plotted against the amount 

 of antibody, three general types of curve are found. The first, 

 which holds for T-^ phage (Hershey, Kalmanson, and Bronfen- 

 brenner, 1943), is a semilogarithmic relation of the form 



In — = —JuA 



where h/hq is the surviving fraction, A is the amount of antibody, 

 and A'4 is a constant. Viewing this process as due to a progressive 

 covering of surface by antibody until the virus is inhibited, it 

 is possible to estimate how many antibody molecules are needed 

 for n/uo to be 37%, which represents a unity chance of virus 

 inactivation. For T-2, the number found is 90. 



The second type of curve shows an initial logarithmic be- 

 havior, but it flattens to a constant remaining activity. This 

 is reminiscent of an equilibrium, but is not so. If further phage 

 is added, precisely the same curve is obtained and the equilibrium 

 point is not shifted. This type of relation holds for T-1. A simple 

 explanation, given more for illustrating the type of process 

 rather than claiming it to be true, is to suppose that two kinds 

 of antigen exist. If one kind of antibody is predominently formed, 

 and some cross reaction with the second antigen exists, then two 

 rate constants would be found. 



The third type is the same but is actually a reversible com- 

 bination. This holds for influenza. 



Surface Inactivation by Antibody 



The figure of 90 molecules for T-2 inactivation is interpreted 

 by supposing that a certain fraction of the virus surface is 

 essential for its multiplication. This can be phrased by saying 

 that some bacterial receptors exist on the virus which are impor- 



