PNEUMOCOCCAL VACCINES 493 



there appeared to be some correlation between the amount of hu- 

 moral antibodies and the degree of resistance, the exception in the 

 case of animals treated with Type III agents, and individual dif- 

 ferences noted in animals as far as this parallelism was concerned, 

 led the authors to conclude that, in addition to differences in the 

 antigenic capacity of strains of various serological types, one 

 must also consider the operation of cellular as well as of humoral 

 immunity in resistance to infection. 



In the experiments of Ross already cited, antibodies appearing 

 in the serum of human beings after the ingestion of pneumococcal 

 vaccines disappeared in some individuals within six months, and in 

 others endured for approximately two years. Felton, Sutliff, and 

 Steele 434 found that the presence of immune substances in the blood 

 of human subjects, injected subcutaneously with the several prepa- 

 rations selected, was appreciable when the serum was tested three 

 months after the immunizing treatment. However, the observations 

 just quoted do not necessarily denote that resistance to infection 

 persisted for so long a time. 



There would be no object in multiplying the number of refer- 

 ences bearing on the duration of active immunity conferred on man 

 by the administration of vaccinal preparations made from pneu- 

 mococci. Sufficient has been said to indicate that resistance, while 

 increasing soon after injection, as far as the presence of circulat- 

 ing protective substances enters into the reckoning, may be natu- 

 rally destroyed within the body or, at least, fail to be demonstra- 

 ble. The period of active immunity is short as compared to the 

 immunity induced by the injection of toxins or of some filtrable 

 viruses. 



RESULTS FOLLOWING VACCINATION IN MAN 



In discussing the biology of Pneumococcus, it is not the inten- 

 tion of the authors of the present volume to invade the fields of 

 preventive or clinical medicine. Notwithstanding this reservation, 

 in order that the allied subjects of antigenicity and of antibody 



