220 ACTIVE IMMUNIZATION 



The further criterion to which reference has been made in the 

 preceding paragraph is the following: 



When a serum is found to retain in any considerable measure, 

 after it has been heated to 60 C. for ten minutes, its power of incit- 

 ing phagocytosis, we may conclude that "incitor elements" (immune 

 opsonins) have been elaborated in the organism either in response 

 to auto-inoculations, occurring spontaneously in the course of tuber- 

 culous infection, or, as the case may be, under the artificial stimulus 

 supplied by the inoculation of tubercle vaccine. 



The above considerations apply also in the case of other bacterial 

 infections, and in the examination of exudates as well. 



As Wright regarded the opsonic index as an indicator of the 

 degree of immunity which develops as the result of bacterial vacci- 

 nation (which see), he advocated that the dosage and frequency of 

 injection should be controlled by opsonic determinations. Accord- 

 ing to his teachings the injection of a dose of vaccine is followed 

 by a decrease of the opsonins (negative phase), which is of variable 

 degree and duration, according to the amount injected. This is 

 followed by an increase (positive phase) coincidently with which 

 there is a corresponding improvement in the patient's condition. 

 The idea of proper vaccination, then, is to so gauge and interspace 

 the different doses that a negative phase is obviated as far as possible 

 and a "high tide" of increased opsonic content secured. 



It would lead too far to discuss the teachings of Wright in any detail 

 at this place; suffice it to say that nearly all investigators who have 

 busied themselves with his technique have come to the conclusion 

 that the unavoidable sources of error are such that accurate results 

 cannot be obtained. As a consequence its application looses much 

 of its raison d'etre, and at the present time there are few outside of 

 Wright's own circle who are influenced in either diagnosis or treat- 

 ment by the opsonic index. But this failure does not in the least 

 diminish the importance of the principle of bacterial vaccination, 

 a principle which had, however, been firmly established long before 

 the opsonins were discovered. 



It would, of course, be most desirable to possess an index to 

 dosage and frequency of injection in vaccination, but a consideration 

 of what has already been said regarding the aggressive forces of 

 the bacteria will at once suggest that even if it could be possible 

 to estimate the "opsonic index" with accuracy, this alone would 



