ANTIGENIC PROPERTIES 111 



a serum, a high A' value indicating a high titer. Once the A' 

 value of a sample has been determined, it can be substituted in 

 the equation and used to calculate the dilution, D, of this serum 

 required to inactivate any desired fraction of the phage in 

 any given time. The range of inactivation over which the 

 equation is applicable must be determined by experiment. 

 For most purposes an inactivation of 90 to 99 per cent is ade- 

 quate and within the range of first-order kinetics. 



Any phage-antibody system which follows first-order kinetics 

 will necessarily obey the percentage law of Andrewes and Elford. 

 The law is more general, however, since it applies also to phages 

 in serological group 3, which do not follow first-order kinetics. 



Hershey, Kalmanson, and Bronfenbrenner (1943a) found 

 that the inactivation of phage T2 by antiserum followed first- 

 order kinetics, the K value being independent of D, t, and Po 

 up to a concentration of about 10^ phage particles per ml. 

 Above this concentration, aggregation of phage particles by 

 antiserum resulted in an increased A' value. 



Kalmanson, Hershey, and Bronfenbrenner (1942) studied 

 some of the variables influencing the rate of neutralization of 

 phage T2 by antibody. A particularly careful study of the 

 reaction using highly diluted antiserum confirmed the initial 

 lag in the neutralization that had been noted by Burnet. Analy- 

 sis of the convexity of the inactivation curve suggested that 

 between 2 and 3 antibody molecules must react with each 

 phage particle on the average for neutralization to occur. 

 Neutralization of T2 by antibody thus appeared to be a multi- 

 hit process. However, it is now known (Sagik, 1954) that a 

 large fraction of the phage particles in certain stocks of T2 are 

 unable to form plaques when plated directly, but become in- 

 fective during treatment with antiserum. The inhibited par- 

 ticles may also be activated in other ways, for example, by 

 heating or by lowering the salt concentration. Activated 

 stocks showed no sign of deviation from first-order neutralization 

 kinetics (Cann and Clark, 1954). Since allowance must be 

 made for genetic differences in the lines of T2 employed by 



