UNSOLVED PROBLEMS 617 



ments in pneumococci that endow them with invasive powers and, 

 among these elements, the type-specific capsular polysaccharide 

 may be the principal determinant. The reasons for the differences 

 in the virulence characteristic of pneumococcal types, of individual 

 members within the types, and of variant forms may be linked with 

 capsule formation, but that is only one element of this vital char- 

 acter. The unusual virulence of Type III pneumococci for man as 

 compared with animals, and the invasiveness of some strains for 

 rabbits and the harmlessness of others are striking examples of 

 the problems to be studied. There are also marked differences in 

 host response among various animal species and among animals 

 within the species, and here the matter is largely beyond our ken. 

 Are the capsular polysaccharides toxic in themselves, or do they 

 combine with tissue elements to produce poisonous effects or, as is 

 now being discovered in the case of the polysaccharides of certain 

 bacteria of the enteric group, do these carbohydrates, possibly 

 linked with lipids, exist in a still more complex form than any yet 

 isolated? 



METHODS FOR THE PRODUCTION OF ACTIVE IMMUNITY 



Procedures for the creation of active immunity in animals and 

 in man are in a state of development. The ideal immunizing antigen 

 and the most effective scheme for administering the antigen are yet 

 to be found. The polysaccharides of Pneumococcus either alone or 

 in a form to be discovered or invented may prove to be the sought- 

 for agent. The possibilities are manifold. 



Is it the protective antibody which must especially be striven for 

 or are other immune substances also required to build resistance to 

 infection? What part — if any — do heterophile antibodies contrib- 

 ute and, more particularly than we now know, what part do the 

 somatic cells play in protecting the infected host, and do other 

 physiological factors share in creating the immune state? 



The serum of animals immunized by the injection of pneumococ- 

 cal materials, besides bearing specific antibodies, contains sub- 



