THE OPSONIC INDEX 235 



Interpretation, Value and Limitations of the 

 Opsonic Index, — Wright contends that the opsonic 

 index, properly done and interpreted, is a reliable ex- 

 pression of an animal's resistance against infection. 

 The effect of a bacterial inoculation, after a transi- 

 tory drop in the opsonic index, is an increase in the 

 degree of immunity of the inoculated animal against 

 the disease caused by the given bacterium. The pri- 

 mary drop or so-called *' negative phase," presumably 

 due to the absorption of preformed opsonins by the 

 injected bacteria or to the systemic effect incident to 

 the cellular stimulation of the organism for the pro- 

 duction of specific antibodies, may be of very short or 

 prolonged duration, and if of any consequence is 

 characterized by the clinical phenomena, general, focal 

 and local, described in the following chapter. In a 

 few hours, after a proper sized dose, the opsonic index 

 rises in the scale. This ascent is termed the " flow " 

 and may fail to reach the normal or soar well above it. 

 Since the effect is but transient, the index will slowly 

 fall and this descent is called the " backflow " or 

 " ebb " or " negative phase," occasionally following 

 an inoculation, if the indices are determined suffi- 

 ciently often, the curve will exhibit a mild " initial 

 rise " preceding the " ebb " or " negative phase," be- 

 fore the real " flow " or " positive phase " sets in 



