138 IMMUNITY IN PROTOZOAL INFECTIONS 



any particular parasite emj^loyed. Two strains of the same sj^ecies of 

 trypanosome may produce very different results. An animal inoculated 

 with one strain may acquire an infection from which it will recover. 

 It may have developed an immunity and be no longer inoculable with 

 this particular strain, though it is still susceptible to inoculation with 

 another strain of the same species. On this account it is exceedingly 

 difficult to differentiate species of trypanosome by what have been termed 

 immunity experiments. 



The mechanism of these various phenomena are far from being properly 

 understood, and it appears that a real explanation will never be obtained 

 till the biochemist has obtained more information regarding the chemistry 

 of the living cell and the fluids to which it gives rise. 



IMMUNIIY IN PROTOZOAL INFECTIONS. 



Immunity in connection with parasitism amongst the Protozoa will 

 be referred to below in connection with individual parasites, but it will 

 be necessary to discuss more fully some of the general features which 

 have just been mentioned above. 



NATURAL IMMUNITY.— As remarked above, each parasite has its 

 own particular host or group of hosts in which it can live, and outside 

 these limits it is impossible for it to establish itself. This specificity, as 

 it is called, is well illustrated by the malarial parasites of man. Exactly 

 how infections are prevented in one host while they take place readily 

 in another is not properly understood, but, as a result of extensive re- 

 searches, it is evident that cells and fluids of the body of refractory 

 animals are of such a nature that parasites introduced cannot develop 

 and are finally killed. That the serum of the blood is largely responsible 

 for this natural resistance is proved by the experiments of Laveran (1904ft), 

 who showed that the blood-serum of baboons, which are usually refractory 

 to inoculation with Trypanosorna gam.biense, when injected into mice will 

 cause the disappearance of T. gambiense from their blood, or even prevent 

 infection if injected forty-eight hours before inoculation with tlie trypano- 

 some. Such an immunity against infection is a natural immunity. It 

 is possible, however, in some cases to overcome the natural resistance. 

 This may be effected either by lowering the resistance of the inoculated 

 animal, an illustration of the well-known fact that a person in good health 

 is less liable to disease than one who is in poor condition, or by increasing 

 the virulence of the parasite. As a rule mice and guinea-pigs are quite 

 refractory to inoculations with Trypanosoma hwisi of the rat, but Eoudsky 

 (1910ft, 1911), as will be mentioned below, was able to increase the viru- 

 lence of the trypanosome, so that mice and guinea-pigs were susceptible. 



