IMMUNITY AND SUSCEPTIBILITY. 91 



duce very little, if any, separable toxic substance, and 

 seem to depend for pathogenesis upon vigor of growth in 

 the channels of the body. Others produce non-specific 

 irritating substances which react so similarly upon the 

 organism that certain processes, as suppuration, fever, 

 etc. , may result from the operation of any one of a con- 

 siderable group of bacteria. Still others produce toxic 

 substances whose operation is truly specific in that they 

 act selectively upon the body cells, always affecting like 

 cells in the same manner, producing similar symptoms. 

 The best illustration of the micro-organisms of this group 

 is the tetanus bacillus, whose toxin, acting upon the 

 motor cells of the nervous system, produces the charac- 

 teristic spasms of the disease. 



Immunity consists not only of overcoming the bacteria 

 that cause disease, but in enduring and annulling their 

 toxic effects. It is found that when the pathogenic bac- 

 teria and their poisons are separated, the immune animal 

 suffers no more effect from the one than the other. Thus, 

 the rat is immune to diphtheria. Infection with living 

 diphtheria bacilli will not kill it, and injection of power- 

 ful toxin in considerable amounts does not injure it. The 

 disease undoubtedly depends upon the diphtheria toxin to 

 which the rat is immune, and for which reason the diph- 

 theria bacillus is harmless to it. Fowls are immune to 

 tetanus and will not succumb to infection. If, however, 

 they are injected with large doses of strong tetanus toxin, 

 they may die. In this case the fowls are able to annul 

 the effects of as much toxin as the bacilli can form in 

 their bodies, but are not able to dispose of unlimited 

 quantities experimentally introduced. 



Immunity to disease, therefore, signifies immunity to 

 the poisons causing disease, and can only be successfully 

 studied in association with the correlated phenomena of 

 intoxication. The reactions brought about in the body 

 by the poisons of bacteria are similar to those caused by 

 the toxalbumins of the higher plants, ricin, abrin, etc., 

 and to some of the animal poisons, such as the venom of 



