CHAPTER XLIX 



THE IMMUNOLOGICAL BASES FOR DIFFERENT TYPES OF 

 INFECTION BY THE BLOOD PROTOZOA 



WILLIAM H. TALIAFERRO 



University of Chicago 



Certain of the blood protozoa by virtue of their easy accessibility and large size 

 offer the unique opportunity of correlating the type of the infection manifested by the 

 parasite with the immunological resistance acquired by the host. Certain prerequi- 

 sites however are necessary before a given species can be used in such a study — the 

 parasite must occur in the peripheral blood of the host in all stages and in sufficient 

 numbers to be studied and the host itself must be somewhat tractable and procurable. 

 For these reasons work in this field has been limited to certain of the trypanosomes 

 and malarial parasites in laboratory animals, but it is to be hoped that in the future 

 other infections may become available. Even some species of trypanosomes and 

 malarial organisms are unsuitable. For example. Trypanosoma cruzi, the causative 

 agent of Chagas' disease, passes a large part of its life-cycle in the vertebrate host 

 as an intracellular leishmaniform parasite and Plasmodium falciparum of estivo- 

 autumnal malaria undergoes the major portion of its asexual reproduction within the 

 smaller blood vessels of the deeper organs. 



The term ''resistance" as applied to the defense of an organism against invading 

 parasites has many diverse usages and meanings. In the present review, where a 

 correlation is being developed between the effects of the host's resistance and the 

 types of infection, the term will be used only for those conditions either active or 

 passive which may arise in the body as a result of infection and which directly affect 

 the parasite. Used in this sense it will not include various possible mechanisms such 

 as defense agauist the act of invasion, regenerative reactions of the host, or the forma- 

 tion of antitoxic substances. Similarly it limits discussion to acquired resistance be- 

 cause, as yet, experimental data are too meager for a systematic study of the effects 

 of natural resistance. 



As a matter of convenience in presenting the present resume I shall give first the 

 various methods which have been developed for analyzing the effects of the host's 

 resistance against the parasite and then under five selected types of infection I shall 

 outline in sequence the course of the infection, the effect of the host's resistance on the 

 parasite, and the immunological bases for the observed effects. 



METHODS OF ANALYZING THE EFFECTS OF RESISTANCE 



After protozoa such as are considered in this review have successfully invaded a 

 vertebrate host they reproduce asexually either by binary fission or schizogony. The 

 first method is almost universally found in the trypanosomes (Figs, i and 2), although 

 in Trypanosoma lewisi at the height of reproductive activity complete division of the 



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