248 Chapter VIII 



the acquired immunity in this case. We have fuller information as to 

 the essential phenomena observed in the organism of the rat vacci- 

 nated against Try2)ano8oma leivisi. We owe to Mme. L. Rabinowitsch 

 and Dr Kempner^ the first important data as to the possibility of 

 immunising white or piebald rats against the disease produced by the 

 flagellated infusorian. They noted that these animals when inocu- 

 lated with the blood of grey rats containing Trypanosomata acquire 

 a very transitory disease w hich, how ever, confers an immunity against 

 any subsequent infection. The flagellated organisms disappear from 

 the blood within a few weeks, after which fresh injections of these 

 parasites have no pathogenic effect. 

 [261] Laveran and MesniF conflrmed these observations, and in ad- 

 dition made careful observations on the mechanism of this acquired 

 immunity. After making several inoculations with blood containing 

 Trypanosomata into white rats, they made a study of the i)roperties 

 of the blood serum of these immunised animals. First they esta- 

 blished the fact that this serum exerts no microbicidal action on 

 the Trypanosomata, but it agglutinates them without, however, 

 rendering them motionless: — "The masses may be resolved into 

 rosettes in which the Try2Kmosomata, united merely by their poste- 

 rior extremities, have their flagella free and motile at the periphery." 



Laveran and Mesnil then studied the phenomena evolved in the 

 refractory organism. When injected into the peritoneal cavity of 

 immunised rats the Tryiyanosomata are not acted upon injuriously by 

 the body fluids. They are, however, devoured by the leucocytes. 

 Laveran and Mesnil thus express themselves on this subject : "...we 

 have demonstrated clearly and repeatedly that the Trypanosomata 

 are ingested alive, perfectly isolated and very motile, by phagocytes, 

 and we have followed the details of this process of ingestion which 

 recalls that of the ingestion of spirilla by the leucocytes of the 

 guinea-pig. We consider, therefore, that the immunity is phagocytic 

 in character." 



The main facts on acquired immunity established in connection with 

 the most diverse micro-organisms, facts just described, may already be 

 said to lead to certain general conclusions. They indicate in the 

 first place that acquired immunity is accompanied by phenomena 

 more complicated than those observed in natural immunity. In the 

 two categories of processes observed in acquired immunity the pha- 



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



2 Ann. de VInst, Pasteur^ Paris, 1901, t. xv, p. 673. 



