OPSONINS 161 



an index below .8 is therefore almost diagnostic of 

 the presence of an infection with the organism tested. 

 Application of the Opsonic Measurements. At 

 the present time Wright has correlated all his obser- 

 vations and built up a system of treating bacterial 

 infections by means of active immunization con- 

 trolled by opsonic measurements. The principles 

 underlying his method may be briefly summarized 

 as follows: In localized bacterial infections the 

 infected body absorbs but small amounts of bacterial 

 substances or antigens. In consequence of this the 

 amount of active immunity developed is but slight. 

 Localized infections therefore tend to run a chronic 

 course. The logical method of effecting a cure in 

 these cases is to actively immunize the body with 

 the invading organism. In a number of infections, 

 notably those of staphylococcus, streptococcus, and 

 tubercle, the degree of immunity is measured accu- 

 rately by the opsonic index. Following an inocu- 

 lation with the infecting bacteria (dead cultures in 

 salt solution) there is first a drop in the opsonic 

 index, the " negative phase," then, depending on 

 the size of the dose and the reacting power of the 

 individual, there comes a rise of the index, the 

 " positive phase," or a continuation of the negative 

 phase. The former is obtained with proper dosage ; 

 the latter with doses too large or too small. In 

 estimating the size of the dose given, Wright counts 

 the number of bacteria per c.c. of emulsion injected. 



