314 INFECTION AND IMMUNITY 



tube. Wassermann and Citron believe that Bail's aggressins are noth- 

 ing further than endotoxins which divert antibodies from bacteria 

 themselves, by neutralizing them. We ourselves, reasoning from work 

 done by the writer with Dwyer, are inclined to believe that Bail's 

 aggressins are of the nature of proteotoxins or anaphylatoxins which are 

 formed in the reaction between bacteria and active serum constituents 

 when phagocytosis for some reason is inhibited. Whether or not the 

 agglutinating and precipitating functions of immune sera have any 

 protective importance is somewhat obscure. 



The recent work of Bull seems to indicate that bacteria are rapidly 

 agglutinating in the animal body and that by this agglutination phago- 

 cytosis of the bacteria, by fixed and mobile phagocytosis throughout 

 the body, is facilitated. 



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 mechanisms of dif- 

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

 other in the natural classification. An explanation of reactions to a 

 given infection which applies in the case of one species is not, therefore, 

 obviously applicable in the case of another species. This is true not only 

 of the mechanism of protection as it takes place in .the serum of dif- 

 ferent animals and in their plasma, but also of phagocytosis and phagocy- 

 tic digestion and the factors which contribute to the perfection of these 

 processes. The constant stumbling-block in the way of a correct in- 

 terpretation of processes going on in the animal body is our inability, as 

 we have seen, to argue from serum phenomena to phenomena occurring 

 in the plasma. A failure to keep this in mind, although it is fully recog- 

 nized, has undoubtedly led to many hasty conclusions, particularly 

 connected with the theory of lytic immunity. This may be illustrated 

 by a well-known example : Fresh rabbit serum is actively germicidal for 

 anthrax bacilli, dog serum is not; yet rabbits are extremely sensitive 

 to a true anthrax infection, while dogs are very resistant. Experiment 

 has shown that there are sensitizers in the sera of both these animals, 

 but that the dog's serum does not contain the complement necessary 

 for their action on the bacilli; the complement presumably has re- 

 mained in the body cells, whereas in the case of the rabbit it has pos- 

 sibly been liberated from the leucocytes during clotting. The reason 

 the dog is insusceptible is, then, not because of plasma destruction of 

 the invading anthrax germs, but probably because of a more perfect 



