NATURE OF IMMUNITY. 355 



low affinity between cell receptors and toxin so 

 that the latter does not unite with the cells except 

 under special conditions (e. g., the immunity of 

 chicken to tetanus) ; or (c) on the presence of 

 natural antitoxins. 



2. Acquired active antitoxic immunity depends 

 on the multiplication and excretion of cell recep- 

 tors (antitoxin) into the circulation, the new- 

 formed bodies having the power of combining 

 chemically with additional toxin which may be in- 

 troduced. 



3. Passive antitoxic immunity, as established by 

 the injection of an antitoxin, depends on the abil- 

 ity of the antitoxin to combine chemically with the 

 toxin and thus to divert the latter from the cells. 



4. Natural immunity to bacteria depends on (a) 

 a lack of suitable cell receptors with which the 

 toxic bacterial constituents might combine; (b) a 

 very low affinity between cell receptors and the 

 toxic bacterial constituents; or (c) on the presence 

 of natural bacteriolysins (amboceptors and com- 

 plements). 



5. Acquired active antibacterial immunity de- 

 pends on the multiplication and excretion into the 

 circulation of specific cell receptors (amboceptors) 

 which have the power of uniting with complement 

 to kill the micro-organisms which may be intro- 

 duced. 



6. Passive antibacterial immunity, as estab- 

 lished by the injection of a bacteriolytic serum, 

 depends on the ability of the amboceptors con- 

 tained in the serum to unite chemically with the 

 receptors of the micro-organism, as a result of 

 which complement is absorbed to kill and perhaps 



