170 PRINCIPLES OF BACTERIOLOGY 



increasing this affinity by means of an excess of antitoxin, a point of 

 considerable practical value in serum therapy. 



Antibacterial Action. THE ACTIVE SUBSTANCES IN BACTERICIDAL 

 AND BACTERIOLYTIC SERUM. The first important fact noted which 

 suggested that a bactericidal serum produced its action through more 

 than one substance was the discovery that while the power of a fresh 

 bactericidal serum to kill bacteria in the test-tube is lost by heating it to 

 60 C., yet injected into the peritoneal cavity or blood of a living animal 

 the power is still exerted. The same is true if in the test-tubes to the 

 heated serum there is added some fresh normal serum which is itself 

 incapable of destroying the bacteria. From these observations the fact 

 became gradually apparent that the bactericidal property of a cell-free 

 serum depended upon two components. 



Different investigators have applied to them different names. The 

 one which is resistant to heat, which attaches itself directly to bacteria, 



FIG. 69 



Graphic representation of amboceptor or receptors of the third order and of complement, showing 

 on left the immune body uniting complement to foreign cell and on right the action of anticomple- 

 ment, binding complement: A, complement ; B, intermediary body; C, receptor; D, cell; E, anti- 

 complement. 



even at low temperatures, and is increased during immunization, is 

 called sensitizing substance, interbody, amboceptor, or immune body. 

 The other, which is sensitive to heat, which is present in the healthy 

 normal serum, is not increased during immunization, and which unites 

 with the bacterial protoplasm only at temperatures considerably above 

 the freezing point, is called alexin, or complement. 



The immune body attaches itself to the bacterial substance, but does 

 not appreciably harm the cells. The complement destroys the cell after 

 the immune body has made the cell vulnerable. 



According to Ehrlich the immune body first unites with the proto- 

 plasm of the cell and this develops in the immune body an affinity for 

 the complement and the two unite. (See Fig. 69.) He believes that 

 it is through the immune body that the complement exerts its action 

 on the cell. 



Others believe that the immune body and complement do not directly 

 unite. It appears as if the immune body injured the cell membrane 



