CHAPTER IX 

 IMMUNITY 



Immunity from disease is dependent upon innumerable extrinsic and intrinsic 

 factors, many of which are not constants. Hence immunity is seldom if ever 

 absolute, it is relative or conditional or partial; it varies in different geo- 

 graphical locations, in different individuals under similar circumstances; in 

 each of us it ebbs and flows like the tide, changes from hour to hour and con- 

 spicuously varies at different age periods of life in infancy, youth and old age. 



We are more or less familiar with some of the extrinsic and intrinsic factors 

 that establish and destroy immunity and there are others of which we know 

 nothing. 



Of all the ailments afflicting man half, or more, are infectionsdiseases in 

 which the body is assaulted by microorganisms or their products. 



The two most important factors in every case of infectious disease are first, 

 the animal afflicted, and second, the offending microorganism. For infection to 

 develop each of these factors must fulfill certain requirements and we may, for 

 convenience of study at first, consider them separately. 



PROPERTIES PECULIAR TO PATHOGENIC BACTERIA 



Bacteria potentially capable of causing disease are those that produce aggres- 

 sin, endotoxin, ectotoxin one, two, or all of which can propagate in the host. 



Aggressin. Having inoculated animals with the typhoid bacillus and so 

 produced a serous exudate, Bail collected this exudate, freed it of bacteria by 

 centrifugalization and mixed a small quantity of it with a sublethal dose of 

 typhoid bacilli. By injecting this mixture into experimental animals he pro- 

 duced death. 



Bail's explanation of this phenomenon is substantially as follows: Pathogenic 

 bacteria, upon entering a host, elaborate a substance that protects the bacterium 

 and assures its development by paralyzing or destroying the bacteriocidal sub- 

 stances of the host which are attracted to the atrium of infection, especially 

 those substances that effect phagocytosis. 



This bacterial emanation, capable of inhibiting phagocytosis, he named 

 aggressin. 



Sterile serous exudates containing aggressin are of themselves poisonous; 

 heating for i hour at 6oC. does not impair their toxicity; by repeated injections 

 of small quantities of them a specific immunity to the aggressin and bacterium 

 producing it can be conferred upon experimental animals. 



Endotoxin. By various experiments, not necessary to describe here, one 

 can demonstrate that all pathogenic bacteria produce a water- and serum-solu- 

 ble substance that is retained within the bacterium until it begins to involute or 



259 



