FACTS AND PROBLEMS OF IMMUNITY 313 



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 probably 

 the endotoxins opposed by leucocytes, and possibly directly in the 

 plasma 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 secretions 

 which oppose the opsonins, and thus prevent phagocytosis — antiopsonins 

 — bodies which may possibly be the so-called aggressins of Bail, and 

 which are present in exudates and, although not toxic in themselves, in- 

 crease the infectiousness of the bacteria with which they are injected; 

 and, finally, opposed to bacteria and their broth filtrates we have the 

 agglutinins and precipitins, the activities of which are manifest in 

 serum, but whose relation to immunity is not altogether obvious, as 

 they have not been shown satisfactorily to bring about agglutination or 

 precipitation in the animal body. 



While all of these different functions and chemical substances are 

 possessed by animals as a class, it is becoming more and more obvious 

 that these are not always present or active in the same degree, and that 

 there are recognizable differences in the protective mechanism of dif- 

 ferent animal species — in species, in fact, not far removed from each 



