CHAPTER XI 



OPSONINS 



Historic. Although there can be no doubt as to the importance of 

 phagocytosis in the mechanism of recovery from infection, yet it was 

 shown by Metchnikoff, as early as 1893, that the body-fluids contained 

 substances that greatly facilitated the phagocytic process, and that 

 leukocytes removed from this influence were practically powerless to en- 

 gulf and destroy the invading bacterium. In other words, if leukocytes 

 and bacteria are washed free from all traces of serum and then mixed, 

 very few of the leukocytes will be found capable of phagocytizing the 

 bacteria, which means that spontaneous phagocytosis is feeble and hence 

 of slight importance. When, however, fresh serum is added, especially 

 the serum of an animal immunized against the microorganism used in 

 the experiment, phagocytosis is marked, and, indeed, most impressive. 

 Metchnikoff attributed this difference to the influence of a substance 

 in the serum that stimulated (stimulins) the leukocytes to become phag- 

 ocytes, but later researches have shown that this is probably erroneous, and 

 that the serum facilitates phagocytosis not by exerting a stimulating influ- 

 ence upon the leukocytes, but by preparing the bacteria for the process by 

 making them, as it were, more attractive to the leukocytes. 



Denys and Leclef, in 1895, were among the first to demonstrate 

 the effect of serum on bacteria in the process of phagocytosis, and the 

 fact that the active substance was not bactericidal in action, but in the 

 nature of a new antibody. Since Metchnikoff had shown that freshly 

 isolated or virulent strains of bacteria were not readily phagocytized, 

 but seemed to resist or repel the leukocytes, it was natural for these 

 observers to suggest that the action of this substance in serum was to 

 neutralize the exotoxins and endotoxins of microorganisms that were 

 regarded as responsible for negative chemotactic influences, and thus, 

 by robbing them of at least two defensive weapons, prepare them for 

 phagocytosis. 



The subject remained in an uncertain state until 1903, when Wright, 

 and later Wright and Douglas, demonstrated more clearly this action of 

 serum upon bacteria in aiding phagocytosis. Using their own modifica- 

 tion of the technic devised by Leishman for measuring the phagocytic 



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