SUSCEPTIBILITY AND IMMUNITY. 251 



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, and 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- 

 gation of bacilli, then, apart from the fact that here but one set of cases is 

 embraced, it can be determined that this force is too feeble it can be seen 

 that, during the active growth of the anthrax organism in the blood, the 

 elongating chains of bacilli curve in and out between the corpuscles, but 

 never penetrate the cells. 



From another side the objection has been formulated that in many cases 

 the organism gets rid of its invaders without the aid of the phagocytes. 

 According to those who support this objection, this happens in the anthrax 

 of pigeons (Czaplewski) and of refractory rats (Be"hring, Franck), in symp- 

 tomatic anthrax of various refractory animals (Rogowicz), and in the septi- 

 caemia of vaccinated guinea pigs, due to the Vibrio Metschnikovi (R. Pfeif- 

 fer). A reexamination of the cases here adduced has, however, shown 

 that in each a very considerable phagocytosis can be proved, and that the 

 negative results of the above observers have been, due to insufficient methods 

 of observation. 



While accepting that the phagocytes do truly absorb the microorgan- 

 isms, other opponents of the theory have urged that these cells are only 

 capable of including microorganisms already killed by other means, and 

 that living microbes are solely to be found within the cells in those cases 

 where there has been a fatal ending in tuberculosis, mouse septicaemia; and 

 so on. Against this may be brought the fact determined by Lubarsch, that 

 the phagocytes of several animals, refractory to anthrax, take up living ba- 

 cilli that have been injected, with greater eagerness than they include those 

 which have been killed before injection. But, further, this objection may 

 be disposed of by direct observation of bacteria undergoing development 

 from within the interior of phagocytes after the latter have oeen destroyed 

 by a substance which is at the same time a favorable medium for bacterial 

 growth as, for instance, beef broth. Such observations have been made 

 upon pigeons rendered immune to anthrax. 



During the last year or two great stress has been laid upon the fact that 

 the body humors themselves possess most marked bactericidal properties, 

 and, in fact, against the theory of phagocytosis has been brought another, 

 baoed upon this power of the humors to destroy the microorganisms. Ob- 

 server after observer has remarked that in blood plasma, defibrinated blood, 

 blood serum, and in the blood as a whole, in the removed aqueous humor 

 and other fluids and exudations of the body, many species of bacteria perish 

 after a longer or shorter interval ; and forthwith an endeavor has been 

 made to find in these facts some elucidation of the phenomena of immunity. 

 Yet the more deeply one examines into the question the more one is con- 

 vinced that no relationship exists between the two. Thus it happens often 

 that the bactericidal property is more developed in susceptible species than 



