132 INFECTION AND IMMUNITY. 



Antibacterial Although such diseases as typhoid and cholera 



Immunity. . n _ 



are accompanied by pronounced toxic symptoms, 

 the poisonous substances seem to be integrally as- 

 sociated with the bacterial protoplasm and not 

 secreted in a soluble or diffusible form by the liv- 

 ing cell ; they are spoken of as intracellular toxins 

 or endotoxins. Observations point to the belief 

 that the endotoxins are liberated only after the 

 bacteria are killed and dissolved. When one, 

 through infection, has acquired immunity to ty- 

 phoid or cholera, his fresh serum is able to kill the 

 respective bacterium, but apparently is not able to 

 neutralize its toxic substance. Hence, on the basis 

 of the nature of the serum, immunity to such dis- 

 eases is spoken of as antibacterial rather than 

 antitoxic. 



Although the subject is still in a developmental 

 state, the conditions indicate that there are two 

 factors in antibacterial immunity., one in which 

 the plasma or serum has a marked, power of kill- 

 ing the micro-organisms, the other in which the 

 destruction is largely by means of phagocytic cells. 

 The former was just referred to. Both agencies 

 operate in many infections, as in typhoid, cholera 

 and others; whereas in other instances (strepto- 

 cocci, staphylococci) phagocytosis is the only de- 

 tectable protective agency. In most cases phago- 

 cytosis cannot take place until certain constituents 

 (opsonins) in the plasma or serum have "sensi- 

 tized" the bacteria. In case of recovery the power 

 of phagocytic destruction is usually enhanced. We 

 have, therefore, to recognize phagocytic immunity 

 as a type, or at least as an important factor, in 

 both nitural and acquired resistance. 



