THEORIES OF IMMUNITY 129 



which draw phagocytic cells to them. 1 Virulent organisms of the 

 same strain not only do not appear to attract leukocytes, but they 

 appear to repel them. Bordet explained the increase of virulence of 

 bacteria through passage in experimental animals on the ground that 

 the less virulent individuals were engulfed and killed; the more viru- 

 lent members survived and produced a thoroughly virulent strain. 

 Vaillard and Vincent 2 and Vaillard and Rouget 3 showed that bacterial 

 toxins may repel or paralyze leukocytic activity; if tetanus spores 

 are bathed with tetanus toxin before injection into the animal body, 

 the leukocytes do not collect at the point of injection, the spores ger- 

 minate and the animal dies of tetanus. If, however, the spores are 

 washed free from tetanus toxin and then injected, leukocytes appear 

 at the site of inoculation, engulf the spores, and either destroy them 

 or prevent their germination. 



The mechanism of chemotaxis has been a subject of much discus- 

 sion. Evidence is accumulating which would suggest that chemo- 

 tactic stimuli of bacterial origin which reach leukocytes enter the 

 phagocytic cell in greater concentration on that side which is nearer 

 the source of the chemotactic substance, lowering the surface tension 

 at that point. A flow of protoplasm in this direction, in obedience 

 to the lowered resistance, will result in the protrusion of a pseudo- 

 podium, w r hich will continue to advance until the surface tension is 

 equalized. 4 This generally occurs when the leukocyte has flowed 

 around or engulfed the organism. 



Engulf ment. The earlier view associated the protrusion of pseudo- 

 podia and the subsequent engulf ment of bacteria or other cell as an 

 autovoluntary act of the leukocyte. The inclusion of inert particles, 

 as dust or other minutely comminuted granules, would appear to 

 discredit this hypothesis. The engulfment of living or inert bacteria 

 or other minute bodies is, as Wells aptly expresses it, 5 " but an exten- 

 sion of the phenomena of chemotaxis. When the substance toward 

 which the leukocyte is drawn is small enough, the leukocyte simply 

 continues its motion until it has flowed entirely about the particle." 



Digestion. The ultimate solution of engulfed substances other 

 than purely inert particles is by intracellular enzymes contained within 



1 Inert particles, as coal dust, are engulfed by phagocytic cells; it is difficult to explain 

 this phenomenon on the basis of chemotaxis. 



2 La semaine medicale, 1891, xi, No. 5. 



3 Ann. Inst. Past., 1892, vi, No. 6. * 



4 See Well's Chemical Pathology, 1914, 2d ed., pp. 230-251 (Saunders & Co.), for 

 an excellent resume of the literature. 



* Well's Chemical Pathology, 1914, 2d ed., p. 238 (Saunders & Co.). 



