482 BIOLOGY OF PNEUMOCOCCUS 



cation of the same biological agents in the case of human beings. 

 Cecil, 202 with his associates, Blake and Steffen, 206 ' 212 ' 214 " 5 in a se- 

 ries of studies on the immunization of monkeys with different pneu- 

 mococcal vaccines administered by various routes, learnt that 

 because of the greater susceptibility of the monkey to pneumo- 

 cocci as compared to man, the test animals failed to respond as 

 readily to the same amounts of vaccine. In order to induce a de- 

 gree of active immunity sufficient to protect monkeys against ex- 

 perimental pneumonia incited with the homologous type of organ- 

 ism, it was necessary to employ small doses of living, virulent 

 pneumococci, or to treat the animals by three intratracheal im- 

 plantations of pneumococcal vaccines (Cecil and Steffen 214 " 5 ). The 

 subcutaneous injection of saline suspensions of killed pneumococci 

 induced a greater amount of protective substances in the vacci- 

 nated animals than did lipovaccine, but in both cases the appear- 

 ance of specific humoral antibodies was not always accompanied 

 by protection against subsequent infection. By the subcutaneous, 

 intravenous, and intratracheal route, three spaced injections of 

 large doses of saline vaccine were required to immunize the animals 

 against inoculation with homologous pneumococci of the four dif- 

 ferent types employed. When the immunizing treatment was car- 

 ried out by way of the trachea, the immunity induced appeared to 

 Cecil to be cellular rather than humoral in character. Large doses 

 were required, no matter by which route given, while intravenous 

 injection of the vaccine resulted in a higher degree of immunity 

 than administration by the other methods tried. 



Cecil concluded that immunity to Pneumococcus, like other 

 forms of immunity, is relative and depends on the capacity of the 

 individual to elaborate antibodies, on the virulence of the invading 

 strain, and on the size of the dose of culture experimentally in- 

 jected, to which might be added the virulence of the culture from 

 which the vaccine is prepared, the amount, the number of injec- 

 tions, and the site selected for the injection of vaccine. Granting 

 that, because of the greater susceptibility of the monkey to pneu- 



