THEORIES OF IMMUNITY 145 



and embryonic connective-tissue cells are more active in chronic infec- 

 tions and have been designated macrophages. 



Although the original explanation of phagocytosis was quite plain 

 and far reaching, subsequent discoveries by the adherents of the " hu- 

 moral theory" showed the potent influence of the body-fluids upon the 

 process. It was demonstrated that without the aid of these fluids 

 phagocytosis is almost negligible. Metchnikoff early recognized this 

 fact, and sought to explain the influence of the body-fluids by assuming 

 that they contained "stimulins, " or substances that stimulated phagocy- 

 tosis. It was likewise shown that the body-fluids contained antibodies 

 and were antibacterial, independent of cells. Metchnikoff recognizes 

 the existence of these conditions, but claims that they are due to the 

 soluble products of phagocytic cells, and thus in a broader sense would 

 maintain the importance of his phagocytes. 



It was soon observed that in certain infections leukocyt.es or other 

 cells, instead of being attracted toward the seat of infection by some 

 unknown chemical stimulus, (positive chemotaxis), were repelled, or at 

 least the attraction was counterbalanced or did not exist (negative 

 chemotaxis). While a satisfactory explanation of these phenomena is 

 still lacking, it may be stated that, in general, the degree of negative 

 chemotaxis is in proportion to the virulence of the microparasite. 



In 1903 Wright and Douglas, and Neufeld and Rimpau threw con- 

 siderable light upon this subject. They demonstrated experimentally 

 that one action of the body-fluids was directed against the microorgan- 

 isms, lowering their resistance, and, making them, as it were, more 

 attractive to the phagocytes, the process of phagocytosis was facilitated. 

 To these substances the name of "opsonins" (from opsono, I prepare for) 

 was applied by Wright, while Neufeld called them."bacteriotropins." 



The leukocytes are not, however, entirely passive and willing to wait 

 until their prey is weakened and fully prepared for their attack. In the 

 presence of an infection they are found to increase, and this leukocytosis 

 is known to be a valuable addition to the defensive forces. They prob- 

 ably also undergo qualitative changes, which increase their antibacterial 

 power. It has been shown that opsonized bacteria attach themselves 

 to the protoplasm of the leukocytes, a physiochemical phenomenon 

 that occurs regardless of whether the leukocyte is dead or alive, although, 

 of course, only the living leukocyte is able to ingest them. 



That phagocytosis is a potent and very important factor in the 

 mechanism of recovery from certain infections is generally admitted, 

 and although it probably has not the far-reaching significance originally 

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