PNEUMOCOCCAL VACCINES 485 



ings, one with the old form of capsular polysaccharide and the 

 other with the new acetylated form, confirmed the observation that 

 the deacetylated type of polysaccharide was antigenic for man, 

 and ascertained that the new form possessed similar immunizing 

 properties. More recently, Felton (1934) 417 tested the antigenic 

 action of the acid-soluble portion of the pneumococcal cell on ex- 

 perimental animals and man, and reported that a small amount of 

 the alcohol precipitate of the acid-soluble fraction immunized the 

 test subjects to a degree approaching that possessed by patients 

 convalescing from pneumonia. In a study with Sutliff and Steele, 

 Felton 434 compared the immunizing effect of various fractions pre- 

 pared from pneumococci with that of vaccines made from the in- 

 tact cell. Although the optimal dose was not established, the injec- 

 tion of two milligrams of the soluble antigen produced as high a 

 protective titer in the human subjects so treated as did the usual 

 single dose of vaccine containing pneumococci. The authors also 

 reported that under certain conditions the antibodies evoked by 

 the soluble antigens prepared from pneumococci of Types I and II 

 were heterologous as well as homologous in type. 



POTENCY TESTS ON VACCINES 



In order to obtain a measure of the immunizing power of pneu- 

 mococcal vaccines, Bengston (1924)" studied the immune effects 

 appearing in mice and rabbits after the administration of prepa- 

 rations obtained in the market. After testing the agglutinin, pre- 

 cipitin, and tropin content of the serum of the test animals as well 

 as their ability to withstand subsequent infection with virulent 

 pneumococci, Bengston concluded that the determination of active 

 immunity in mice gave the best estimate of antigenic strength, par- 

 ticularly of polyvalent pneumococcal vaccines. The superiority of 

 mice over rabbits could be ascribed to the fact that the virulence 

 of the three fixed types can be maintained at a uniform level for 

 the first-named species. In testing separately experimental vac- 

 cines prepared with organisms of Types I, II, and III, it was 



