FACTS AND PROBLEMS OF IMMUNITY 313 



they are inactive for lack of the complement which is normally retained 

 in the leucocytes, and that they simply prepare the bacteria for com- 

 plete digestion in the leucocytes. From the work of recent years it 

 would seem rather unlikely that either alexin or sensitizer take their 

 origin in the leucocytes in the cells. 



In 1894 a further adjustment of differences took place, when certain 

 phenomena observed by Denys and his pupil Leclef demonstrated that 

 the act of phagocytosis when performed in serum, in some instances 

 at least, was dependent on the presence of certain substances in the 

 serum. Thus, they were able to show that leucocytes removed from 

 normal blood and placed with bacteria in immune serum enulfed 

 the bacteria actively, while leucocytes from immunized animals mixed 

 with bacteria in normal serum took up the organisms no more actively 

 than the normal leucocytes. The bodies inciting the phagocytosis must 

 obviously, then, they concluded, be in the serum. Whether these bodies 

 acted on the leucocyte or on the bacteria was not then determined, 

 but Denys concluded, in 1898, that the bacteria were directly affected. 

 The fact that the action is exerted on the bacteria was recently de- 

 termined positively by Wright for normal serum, and by Neufeld and 

 Rimpau, independently of Wright, for immune serum. These bodies 

 have been called opsonins by Wright, and bacteriotropins by Neufeld, 

 and have been shown to attach themselves to the bacteria and thus 

 prepare them for ingestion by the phagocytes. It has also been shown 

 by various observers that the more virulent the germ, the less susceptible 

 it is to phagocytosis and the more potent the antisera must be to permit 

 of the ingestion by the cells. 



If now, for clarity of conception, we summarize briefly the disease- 

 producing agents possessed by the bacteria and the opposing substances 

 of the serum and processes of the animal body, we find the true toxins, 

 including probably leucocidins and hemolysins, opposed by antitoxins 

 which become free in the plasma; the bacterial bodies and possibly the 

 endotoxins opposed by leucocytes, and by lytic substances formed of 

 amboceptor and complement, which either kill or dissolve the bacteria 

 and free the endotoxins, but do not neutralize them; and, third, we 

 have probably certain substances which oppose phagocytosis. 



These last named substances are largely problematical. Bail has 

 described substances which he calls aggressins, which he thinks prevent 

 the destruction of bacteria by protective mechanism of the invaded 

 body. He believes that these are secretions produced by the bacteria 

 under the conditions found in the normal body but not in the tensv 

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