ANTIGENIC PROPERTIES 107 



of binding neutralizing antibody exist elsewhere than at the 

 tip of the phage tail. More work is needed, however, before 

 this interpretation is accepted. 



Burnet also performed cross-absorption experiments using 

 serologically related phages. Absorption with bacteria coated 

 with homologous phage reduced the neutralization titer for 

 homologous and heterologous phages alike, whereas bacteria 

 coated with heterologous phage absorbed antibody against 

 this phage but had little effect on the titer against homologous 

 phage. This type of experiment permits one to make an anti- 

 genic analysis of a series of related phages. Antiphage sera 

 can also be absorbed with phage concentrates, the precipitated 

 phage particles being removed by low-speed centrifugation 

 (Hershey, Kalmanson, and Bronfenbrenner, 1943a, 1943b; 

 Hershey, 1946a). An advantage of the use of phage-coated 

 bacteria is that the adsorption of the phage particles to the bac- 

 teria serves to concentrate and purify them in one step. It 

 should be kept in mind, however, that those phage antigens 

 which are located at or near the site of adsorption to host cells 

 may not be accessible to antibody. 



8. Specific Soluble Substances 



The first evidence for a soluble substance having the antigenic 

 specificity of phage particles was obtained by Asheshov (1925). 

 He filtered a phage preparation through a collodion membrane 

 of a pore size small enough to retain the phage particles. The 

 immunization of rabbits with the phage-free filtrate resulted 

 in the production of specific phage-neutralizing antibody. 

 Burnet (1933b) prepared sterile ultrafiltrates of high-titer stocks 

 of coliphage CI 6 using Gradocol membranes of a suitable per- 

 meability to retain all the phage particles. The ultrafiltrates 

 immunized rabbits with the production of neutralizing antibody 

 for phage CI 6. The ability of the ultrafiltrate factor to react 

 with antibody was most conveniently detected by mixing it 

 with antiphage CI 6 serum and observing a decrease or blocking 



