278 Chapter IX 



development of acquired immunity, but in this case the properties 

 of the fluids of the body are but little in evidence or they may be 

 even nil. Let us return to the example of the Trypanosoma of the 

 rat which excites in vaccinated animals a protective and weakly 

 agglutinative power of the serum. This fluid, however, is usually 

 found to be incapable even of rendering the flagellated parasites 

 motionless. 



The question of immunity against malaria has been much dis- 

 [292] cussed. It is well known that a first attack of this disease, so far 

 from conferring any immunity of the slightest durability, leaves a 

 certain predisposition to another attack. In spite of this the study 

 of malaria in various countries and in individuals belonging to 

 different races has demonstrated that there does indeed exist a 

 certain degree of acquired immunity against this disease. During 

 recent years Koch 1 has paid special attention to this subject and ha 

 furnished us with very valuable data, based especially on a com- 

 parative study of the blood of children and adults. The frequency 

 of Laveran's parasite in the former and its rarity in the latter, have 

 led him to the conclusion that infantile malaria sets up an immunity 

 which persists in the adult. Moreover, it has been established that 

 in malarial countries the indigenous inhabitants exhibit an attenuated 

 form of the disease, unaccompanied by acute attacks, but with phe- 

 nomena that are chronic and very slow in development. 



In spite of the existence of a certain degree of acquired im- 

 munity against malaria, all attempts to demonstrate any protective 

 action of the serum have been fruitless. Celli 2 , indeed, injected, as a 

 preventive, the blood serum of individuals who had recovered from 

 malaria or of others who were bled during the period of defer- 

 vescence following an acute crisis of this disease, but in every 

 instance these injections were found to be useless in preventing an 

 attack of malaria. 



We can readily understand that in a disease which is exclusively 

 human, such as malaria, it has not been possible to perform a 

 sufficient number of experiments to decide the question of the 

 protective property of the blood. In this respect we shall have 



1 Deutsche med. Wchnschr., Leipzig, 1900, S. 781. 



1 " La Malaria secondo le nuove recherche," Roma, 1899, p. 86 [translated into 

 English by Eyre from the 2nd Italian edition under the title " Malaria according to 

 the new researches," London, 1900]. " Die Malaria " [German translation of same] in 

 Behring s " Beitriige z. exper. Therapie," 1900, Bd. i, Hft. 3. 



