ALL ERG I A 87 



cessation in the formation of the amboceptors be explained on the 

 basis of the gradual disappearance of the bacteria. 



On the contrary, it is evident that a stimulus has been given which 

 remains operative long after the primary impulse to amboceptor for- 

 mation has ceased ; to return to our simile, the second tuning fork still 

 vibrates, though the first one which gave rise to its vibration has already 

 become quiescent. The animal has coincidently developed a resist- 

 ance to the organisms in question which is far beyond its original 

 value; it may indeed be absolute so that subsequent infection is 

 altogether impossible. This resistance, moreover, in the case of 

 some organisms at least, may be lasting, e. g., the immunity which 

 follows an attack of typhoid fever or of Asiatic cholera in man. 



If the blood of a recently injected animal (using the typhoid 

 bacillus for example) is further examined it will be found that, aside 

 from the resultant bacteriolytic properties, it has developed still other 

 characteristics which the serum of the untreated animal either did 

 not possess at all, or if so, only to a slight extent. For it will be 

 observed that such blood even though freely diluted, has now the 

 power of causing the arrest of motility and the clumping or agglutina- 

 tion of the corresponding organisms (W idol's reaction}, and this 

 result, like the production of the bacteriolysins, is not dependent 

 upon the introduction of living bacteria, but may be effected with 

 dead organisms as well. If, further, analogous experiments are 

 carried out with organisms like the diphtheria or the tetanus bacillus, 

 it will be observed that still other changes develop in the body of the 

 infected animal and that bodies here appear in the blood serum which 

 have the power of neutralizing the specific poisons formed by the 

 organisms in question. Then, again, a curious reaction develops 

 in animals which have been infected with the tubercle bacillus, for 

 example, for on subsequent injection with certain derivatives of this 

 organism (tuberculin) the animal responds with fever while the 

 previously untreated control shows no reaction whatever. 



Allergia. These various responses in the reaction of the animal 

 to the introduction of bacteria are now recognized as being merely 

 a partial expression of a general biological law, to wit, that the 

 animal organism invariably responds to the parenteral introduction 

 of foreign cells, i. e., the introduction of cells by other channels than 

 through the gastro-intestinal canal, whether these be of animal or 

 vegetable nature, or of the products of foreign cells, in so far at least 



