SERO-THERAPEUTICS 167 



To immunize a guinea-pig against cholera, dead bacilli are injected 

 into the peritoneum, and afterwards living virulent bacilli in gradually 

 increasing doses. By these means the animal is rendered immune and its 

 serum can be used for showing the specific reaction. If about 30 milli- 

 grams of this serum be mixed with a fatal dose of virulent bacilli, and the 

 mixture be injected into the peritoneum, no ill effects are produced. The 

 bacilli are destroyed by the serum. They become clumped together, lose 

 the power of movement, and break up into granules. Outside the body, 

 too, in a hanging-drop preparation under the microscope the process can 

 be seen. The bacilli become motionless, stick together, and undergo 

 granular degeneration. According to Pfeiffer it is strictly a specific reaction 

 which can be used to differentiate the cholera bacilli, no other bacteria, not 

 even the vibrios, behaving in this way. The facts have not been received 

 without some scepticism, and the reliability of the reaction is by no means 

 universally recognized. The isolation of the active substance in the serum 

 has not been accomplished any more than in the case of other antitoxines. 

 Seeing that ' a serum which had been putrefying for months retained its 

 specific activity almost unweakened ' (161), it is evident that the ' anti- 

 bacterial ' bodies must have the stability of mineral substances, and be very 

 different from antitoxines and all other products of the animal body. The 

 facts that the cholera bacilli in a hanging-drop culture finally recover from 

 the paralyzing effects of the serum, and that pure diluted normal serum 

 of pigeons undergoes a very similar granular degeneration, show us that 

 we must be cautious in forming an opinion. The whole phenomenon has 

 a very suspicious similarity to plasmolytic changes, such as might well be 

 caused by the salts of the serum or of the bouillon (Fig. 6, p. 8). 



The observations cited do not, of course, embrace all that a theory of 

 immunity has to go upon, but they are the chief data. By immunity has 

 always been meant insusceptibility to a disease — the power of resisting an 

 invading contagium. According to recent discoveries we must distinguish 

 between an immunity to the bacteria and an immunity to their toxines. 

 Furthermore, a distinction must be drawn between natural or congenital, 

 and acquired immunity. Cold-blooded animals, for instance, are immune 

 to the diseases of warm-blooded ones, our domestic animals are immune to 

 cholera, the dog to small quantities of anthrax bacilli. Individual variations 

 occur as might be expected. We see it in man where a personal immunity 

 seems often to exist, and must probably be regarded to some extent as 

 a matter of ' predisposition.' The age of the individual, too, brings changes 

 in the natural immunity, as is shown by the existence of children's diseases. 

 Perhaps these are ' immunity diseases ' intended to prepare and fortify the 

 young mortal against bacterial enemies to come, but the question cannot 

 be discussed here. 



Immunity can only be acquired by a pathological change, be it by the 



