142 IMMUNITY IN PROTOZOAL INFECTIONS 



introduced is only capable of reproducing asexually a certain number of 

 times, and that when this is completed sexual forms are produced. It 

 seems clear that the production of substances Vvdiich are generally termed 

 antibodies in the blood of the host plays some part, for when once a host 

 has passed through an acute infection it is rarely possible to produce as 

 intense infection again, while in many cases a complete immunity to 

 further infection is developed. But the second factor also comes into 

 play, for it has been shown that as one infection is subsiding it may be 

 possible to reinoculate the host with the same organism, so as to produce 

 a superimposed infection. Noller (1917) has shown that frogs which 

 have passed through the acute stage of an infection with Trypanosoma 

 rotatorium may be reinfected, though trypanosomes remaining from the 

 first infection are still present in the blood in small numbers. Such a 

 superimposed infection may become as intense as the first one, and even 

 bring about the death of the host. Similarly in the case of piroplasmosis 

 of cattle, Ed. Sergent and his co-workers (1924) have demonstrated that 

 superimposed infections are possible. They found that the appearance 

 of parasites in the blood after the second inoculation was not accompanied 

 by any of the symptoms which followed the first infection. The animals 

 had been rer^^ered partially immune, so that the injurious effects of the 

 parasite were resisted, though its development was not prevented. In 

 order to distinguish this partial immunity or tolerance immunity from 

 an absolute or true immunity they have introduced the term " premuni- 

 tion." It occurs in the infections with Babesia bigemina. The term is 

 not applicable to infections with Babesia nmtafis, which can also be super- 

 imposed on an already existing infection, for the first infection is not 

 accompanied by any recognizable symptoms. This parasite appears to 

 produce no immunity whatever. Hoare (1923) found that sheep, when 

 constantly infested with keds, always harbour Trijixinosoma melo'pliagium , 

 but if the animals are freed from keds the infection in the sheep gradually 

 subsides, till after two or three months it can no longer be detected. It 

 is evident that the batch of parasites introduced by the keds on one 

 occasion have only a limited term of existence in the sheep, and it wovdd 

 appear that this is dependent rather on what may be termed an exhaustion 

 of the parasite than on changes in the sheep, for infection may at any 

 time be re-established by further introduction of trypanosomes from the 

 keds. This exhaustion, however, may be the result of continued action 

 of the antibodies producing a gradual weakening of the parasite. 



It seems clear that in the case of many human Protozoal infections, 

 such as malaria, trypanosomiasis, and amoebiasis, in localities in which 

 these diseases are prevalent, individuals are constantly being infected 

 with fresh batches of parasites, and a condition resembling that in the 



