IMMUNITY AND ANTITOXINS 249 



phagocyte theory depends largely for its validity upon 

 chemiotaxis, which latter was a property of the products of 

 the bacteria contained in the blood serum. 



5. The Antitoxin Theory. We have gathered, then, that 

 whenever bacteria, introduced into the blood and tissues, 

 fail to multiply or produce infection (as in saprophytic 

 bacteria, or in immunity of a particular animal from a specific 

 microbe), this inability to perform their role is brought 

 about by some property in the living and normal blood 

 serum which opposes their life and action ; and further we 

 have learned that this protective property is exhaustible ac- 

 cording to the number of bacteria, and differs with various 

 species of bacteria, and in different animals. Buchner 

 designates these protective bodies, held in solution in the 

 blood, alexines, and regards them as belonging to the albu- 

 minous bodies of the lymph and plasma. Where the blood 

 and tissues do not possess this power, the animal is suscept- 

 ible. Now, as we have already seen from the experiments 

 of Ogata, Kitasato, and others, the blood of an animal dead 

 of anthrax is protective against anthrax, from which and 

 the foregoing it appears that microbes produce by their 

 growth in the tissues poisonous substances we term toxins, 

 which have the power of producing in the blood and body 

 cells substances inimical to themselves, named antitoxins, 

 and so long as these latter substances remain in the tissues 

 the body remains insusceptible to further attacks of the 

 same disease. Alexines are naturally produced antitoxins; 

 antitoxins are acquired alexines. Hence we have the well- 

 known terms ' ' natural ' ' and ' ' acquired immunity. ' ' Of the 

 former we have already spoken. Acquired immunity is a 

 protection not belonging to the tissues of individuals natur- 

 ally and as part of their constitution, but it is acquired 

 during their lives as a further accomplishment, so to speak, 

 of their tissues. This may happen in one or both of two 

 ways. Either it may be an involuntary acquired immunity, 



