842 IMMUNOLOGY 



Role of Immune Processes in the Development 

 OF Protozoan Infections 



GENERAL METHODS 



The detailed considerations in the succeeding sections deal largely 

 with the role of antibodies and cells in modifying the course of infection, 

 together with such allied subjects as recovery, relapse, and immunity to 

 super- and reinfection. The Protozoa offer the advantage of being large 

 enough so that one can ascertain the effect of these processes in a way 

 that is impossible with the smaller bacterial and virus invaders. This 

 analysis has been further facilitated by selecting certain plasmodia and 

 trypanosome infections in which practically all stages in the life cycle 

 of the parasite are accessible for study (i.e., are more or less evenly 

 distributed in the peripheral blood and are not localized in the deeper 

 tissues). 



The course of these infections can be roughly indicated by changes 

 in the number of organisms. Since, however, the number curve is the 

 resultant of the number of parasites produced by reproduction and the 

 number of organisms which die or are actually destroyed, the only 

 deductions that can be drawn from it are that reproduction is going on 

 if the numbers increase, although the rate may be actually decreasing; 

 and that the rate of reproduction is being inhibited, or that the parasites 

 are being killed, or that both activities may be operating, if the numbers 

 remain constant or decrease. The rate of reproduction, if ascertained, 

 however, in conjunction with number counts throughout an infection, 

 will adequately indicate whether the host acquires a defensive mecha- 

 nism, and, if so, whether it is directed toward inhibiting reproduction of 

 the parasites, or destroying the parasites after they are formed, or both. 



All valid measures of the rate of reproduction so far devised, which 

 are independent of the number of organisms killed, depend upon some 

 measurement of size (since organisms usually grow before they repro- 

 duce) or upon some determination of division forms. A direct measure 

 can be devised for the plasmodia, which divide more or less synchro- 

 nously; but an indirect measure has to be resorted to for the trypanosomes, 

 since they neither divide synchronously nor can their fission rate be 

 ascertained directly as can be done among the free-living forms. 

 The particular criteria used are of course more or less arbitrarily selected 



