TYPES OF IMMUNITY. 151 



exceeds normal serum in its bactericidal power for 

 the bacillus itself. 



Having ascertained by observation or experi- Determina- 

 ment that a certain species has a degree of immun- Types of 

 ity to an infection, certain lines of investigation 

 may be followed for the purpose of determining 

 the character of the immunity. If the animal 

 fails to become infected following the injection 

 of a living and virulent culture, it is fair to assume 

 that the organisms have been killed within the 

 body of the animal. That this has been the result 

 may, indeed, be determined by microscopic exam- 

 ination of the different tissues and by the inocula- 

 tion of culture media. It is often desirable to 

 determine the extent to which micro-organisms 

 are eliminated through the excretions (urine and 

 feces) ; this is best done by the culture method, 

 but it is often a difficult technical problem. 



The natural antibacterial forces with which we 

 are familiar consist of the germicidal action of 

 the serum and plasma, and the phagocytic and 

 destructive action of the leucocytes, endothelial and 

 perhaps other cells. It is difficult to obtain sat- 

 isfactory results by a study of these forces within 

 the body of the animal, particularly as regards the 

 bactericidal action of the serum and plasma, hence 

 such studies are usually carried on outside the 

 body. Such experiments are open to the criticism, 

 however, that the artificial conditions are far 

 removed from those of the body, and that the 

 results do not always justify us in drawing con- 

 clusions regarding the course of events within the 

 body. Phagocytosis under natural conditions is 

 more susceptible to study, and it is to be remem- 

 bered that Metchnikoff came to his far-reaching 



