SUSCEPTIBILITY AND IMMUNITY. 261 



tures or in caseating masses these changes may well be regarded as due to 

 a specific action upon the part of the giant cells. 



The broad fact that the invasion of the organism by microbes most often 

 induces, on the one hand, an inflammatory reaction with its associated emi- 

 gration of leucocytes, and that, on the other hand, the phagocytes are 

 capable of including and destroying the invaders, leads us to admit that the 

 afflux of phagocytes to the invaded region, and their bactericidal properties, 

 are mechanisms which serve to ward off bacterial attack and to maintain the 

 integrity of the organism. Where the phagocytes do not, either immediately 

 or eventually, intervene, but leave the field free to the microbes, these last 

 multiply without hindrance and succeed in killing the animal within, it may 

 be, an excessively short period. Thus the microorganism of hog cholera, 

 which is left quite untouched, kills the pigeon in the course of a few hours 

 often within five hours after inoculation ; chicken cholera kills not only 

 pigeons but also rabbits in an equally short period. In other diseases in 

 which the phagocytes appear upon the scene in relatively large numbers, 

 and even include the microorganisms, the latter gain the day whenever and 

 wherever the phagocytes are incapable of destroying them or of preventing 

 their growth. 



This manifest bactericidal action is to be compared with the phenomena 

 of intracellular digestion characteristic of amoeboid cells in general, and of 

 leucocytes and other microbic phagocytes in particular. These cells have 

 the power of digesting with ease red corpuscles and other organized ele- 

 ments, just as have the amoebae proper and other protozoa. Among these 

 last are many which have been found to include and transform bacteria in 

 exactly the same way as do the phagocytes of the higher animals. 



Now, in determining the intervention or non-intervention of the leuco- 

 cytes in this war between the organism and the bacteria, a very great part 

 is played by the sensitiveness of these cells to external influences, and es- 

 pecially to the chemical composition of their environment. The leucocytes 

 are powerfully attracted by many microorganisms and the resultants of 

 their growth, and as powerfully repelled by others and their resultants, or, 

 as it is expressed, they have a positive chemiotaxis for certain microbes, a 

 negative chemiotaxis for others. The existence of these chemiotactic pro- 

 perties has been so clearly proved of late by the researches of Leber, Mas- 

 sart and Bordet, and Gabritschevski that I need not enter into a fuller ex- 

 planation of the subject here. Where negative chemiotaxis manifests itself, 

 there, being shunned by the white corpuscles, the parasites freely propagate 

 themselves and induce the death of their host. Nevertheless this chemio- 

 taxis is not immutable, and the cells can become accustomed to substances 

 from which they shrank at first a negative may thus be transformed into a 

 positive chemiotactic state. Such obtains in acquired immunity ; the cells 

 which in the unvaccinated animal never included the bacteria, now in the 

 vaccinated take them up readily. . . . 



There is not a single portion of the theory which I have just expounded 

 but has encountered a lively opposition. Even the fundamental fact that 

 the phagocytes are capable of including the microbes has had doubts thrown 

 upon it ; it has been held that the latter insinuate themselves into the for- 

 mer. Only after successive series of observations upon the phagocytes and 

 the living microbes has it been proved that assuredly it is the phagocytes 

 which, by the aid of their pseudopodia, themselves include the microorgan- 

 isms. The observer can see the whole process in the case of immobile ba- 

 cilli can see the leucocyte approach, send out pseudopodia, arid gradually 

 include the individual bacillus. Or, conversely, in cases of negative che- 

 miotaxis, one can, in blood taken from the monkey during the access of re- 

 lapsing fever, observe the actively moving spirilla come into contact with a 

 leucocyte, and even become attached by one end to its surface ; yet, how- 

 ever active the movement, one never finds that the spirillum succeeds in 

 piercing the surface and gaining an entrance. If it be suggested that this 

 entry may take place in consequence of the force of active growth and elon- 



