316 Chapter X 



the animal kingdom. Such examples are not numerous, as, in the 

 majority of cases of acquired immunity against Protozoan parasites, 

 the serum is inactive and incapable of communicating immunity to 

 normal individuals. We have only a single example, the Trypano- 

 soma of rats, against which Dr Lydia Rabinowitch and Dr Kempner 1 

 have demonstrated the possibility of immunisation by the blood serum 

 of vaccinated white rats. The mechanism of this immunity has been 

 studied by Laveran and Mesnil 2 , who found that it was like that 

 described (Chap. VIII) in connection with the immunity in white rats, 

 conferred by the inoculation of living Trypanosomata. The specific 

 [332] serum does not affect these infusoria except that it brings about slight 

 agglutination. Trypanosomata placed in contact with it retain their 

 pristine vitality and motility. This fact led Mme Rabinowitch and 

 Dr Kempner to advance the hypothesis that the protective action of 

 the serum must depend upon its antitoxic power. Since, however, in 

 the infection of rats by the Trypanosomata, the toxic action is very 

 feeble if not nil, it is very difficult to accept this view. It certainly 

 appears to be much more probable that the serum acts in this case, as 

 in many others, by stimulating the phagocytic reaction. The rapidity 

 with which the living Trypanosomata are ingested by the phagocytes 

 has been shown by Laveran and Mesnil. 



Reviewing the whole of the data on immunity produced under the 

 influence of anti-infective or protective serums, it is evident that they 

 fall under two main categories. On the one hand there is a direct 

 action of these serums on the micro-organisms, an action that is either 

 microbicidal properly so called, agglutinative, or fixative. On the 

 other hand, a stimulation of the phagocytic defence which leads to 

 the final destruction of the micro-organisms is set up. This last factor 

 is general ; even in the case where the direct action is most marked 

 (vibrios in the phagolysed peritoneal cavity), its importance is con- 

 siderable. The micro-organisms which can be deeply injured by the 

 direct action of the specific serum are few in number. In most cases 

 this action is a feeble one and needs, for its completion, effective 

 co-operation on the part of the phagocytes. In this respect micro- 

 organisms present a whole gamut which begins with the cholera 

 vibrio, the micro-organism most sensitive to the action of the body 

 fluids, and ends with the Trypanosoma of the rat, a flagellated 



1 Ztschr.f. Hyg., Leipzig, 1899, Bd. xxx, S. 251. 



8 Laveran, "litres et travaux scientifiques," Paris, 1901, p. 39. Ann. de Vlnst. 

 Pasteur, Paris, 1901, t. xv, p. 673. 



