PRODUCTION OF ANTIPNEUMOCOCCIC SERUM 575 



portions of serum and culture employed. The so-called Schwellen- 

 wert, also noted by Neufeld and Haendel, 989 is another phenome- 

 non which limits the applicability of the test. The Schwellenwert 

 may be defined as the maximal amount of culture against which 

 protection can be demonstrated with any amount of serum. Coven- 

 try (1927), 286 in measuring the protective action of serum by not- 

 ing survival time of the mice, failed to obtain evidence of this 

 threshold of effectiveness of serum. That such a relation exists, 

 however, appears to be generally conceded although the explana- 

 tion is obscure. Goodner and his colleagues 537, 540 believed the phe- 

 nomenon to be due to the interrelation between serum and culture 

 and the number and kind of phagocytic cells in the host. The au- 

 thors also list certain zonal effects as referable to the variable 

 factors in the test. Thus, the limiting zone of titration is defined as 

 the smallest amount of serum that will protect against a given 

 quantity of culture, and is believed to be related to the number of 

 virulent organisms and to the antibody content of the serum. The 

 reaction, however, is not strictly one following the law of multiple 

 proportions. In the same general category is the so-called pro-zone 

 which is manifested by a lack of protection when an excessive 

 amount of serum is employed. The pro-zone is, therefore, related 

 to the Schwellenwert. Goodner and Horsfall 537 found such a zone 

 in tests with immune horse serum but not with immune rabbit se- 

 rum, and others have confirmed this observation. 



The fact that the zonal effect may be observed in tests with 

 horse serum but not with rabbit serum suggests the presence of 

 some non-protective, antagonistic substance in immune horse se- 

 rum. The effect may be at least partially obviated according to 

 Goodner and Horsfall by previous treatment of the mice with so- 

 dium nucleinate. That excessive amounts of specific polysaccha- 

 ride may serve to operate against the protective action of serum 

 is a possibility suggested by the work of Felton and Bailey 

 (1926). 419 The interference may be due simply to a combination of 

 polysaccharide and antibody, thus decreasing the amount of anti- 



