MMUNOLOGY 



pathogenesis has not been worked out for any human 

 protozoon. Experimental studies are possible in lower 

 animals, e. g., amoebiasis in cats, and most of what we 

 know of the pathogenesis of protozoan diseases has been 

 gained in this way, but the complete story for any 

 one of them is not yet possible because of lack of 

 observations. 



(3) Immunology. The subject of immunity to proto- 

 zoan infections is in its infancy and the little we know 

 about it at present is based principally on epidemiological 

 observations and on animal experiments. That hosts 

 differ considerably with respect to natural resistance to 

 various protozoan diseases is evident since many indi- 

 viduals do not become infected although they are un- 

 doubtedly invaded by the parasites. Acquired resistance 

 has been demonstrated in certain cases. 



8. CHANGES IN THE PARASITE DUE TO RESIDENCE IN THE 

 HOST 



(i) Immunology. Residence in a host may bring 

 about the building up of an active, acquired resistance on 

 the part of the parasite in certain cases. The well-known 

 hypothesis of Welch (1902), that bacteria are stimulated 

 to protect themselves by the production of antibodies 

 when they are subjected to the defensive forces of the 

 host, no doubt holds true for protozoa. That this should 

 occur is not at all strange since parasites are living or- 

 ganisms and doubtless react to various stimuli much as 

 does the host, but they appear to be capable of modifica- 

 tions that enable them to resist harmful therapeutic 

 agents. Thus races of parasites are supposed to develop 



35 —->>.- 



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luj ^ L I 3 R A R Y 



