ANTIGENIC PROPERTIES 117 



sessed the serological genotype of one or the other parent, but 

 never of both. Interestingly, the first-cycle T2 X T4 progeny 

 were found to fall into three phenotypic classes : the two parental 

 classes, and a class with mixed serological phenotype (Streisinger, 

 1956c). The quantitative distribution among these classes 

 was independent of genotype (Chapter XVI). 



At the moment, it is not possible to give a unique structural 

 interpretation to the particles, obtained in either the T5 X 

 PB or the T2 X T4 crosses, which resemble both parents serolog- 

 ically. It is tempting to draw the conclusion that they con- 

 tain two distinct tail antigens, corresponding to the distinct 

 parental antigens. They could, however, contain neither of 

 these but a third antigen, which cross-reacts with both anti- 

 bodies. In this respect, serological data obtained with cross 

 progeny are no less ambiguous than data obtained by cross- 

 absorption analysis of related phage strains. This ambiguity, 

 clearly stated for phage workers by Hershey and Bronfenbrenner 

 (1952) and Luria (1953a), is conveniently overlooked by most 

 serologists. But while the heterogeneity of the tail antigen 

 has proved elusive, there can be no question of the heterogeneity 

 of neutralizing antibody. The simple demonstration that ex- 

 haustive absorption of an antiserum with heterologous phage 

 removes part but not all of the antibody for the homologous 

 phage proves that the neutralizing antibody molecules in the 

 serum are not all alike. Unfortunately for structural interpre- 

 tations, it is well known that a single antigen can stimulate the 

 production of a variety of antibody molecules. 



13. Mechanism of Phage Neutralization 



We shall briefly review the observations that seem especially 

 relevant to the mechanism of neutralization by antibody. 



7. Neutralized phage can be reactivated by treatment with 

 proteolytic enzymes or high frequency sound. Hence, antibody 

 molecules do not destroy the phage, but interfere mechanically 

 with infection by their presence at the phage surface. 



2. Although a particle of phage T2 can accommodate a total 



