858 IMMUNOLOGY 



munity, but, since natural immunity may be due to many factors, the 

 only conclusion that is warranted from negative experiments is that the 

 experimental method used did not disclose any natural immunity. With 

 these facts in mind, we may conclude the following: Guinea pigs seem 

 to possess a high natural parasiticidal immunity to T. leivisi, since T. 

 leu'isi hardly increases in numbers at all in spite of the fact that it goes 

 through the same reproductive cycle as it does in the rat. Mice seem 

 to possess a high natural ablastic immunity to T . duttoni, since T. duttoni 

 in splenectomized and blockaded mice reproduce at a higher rate than 

 in normal mice. Rats have not been shown to possess any natural im- 

 munity against T. leivhi. 



The drop in numbers at the crisis (at location 2, in Fig. 189) repre- 

 sents the acquisition of a trypanocidal response on the part of the host, 

 in addition to the ablastic effect, since if only the latter were present the 

 numbers would remain constant. Also, the disappearance of the trypano- 

 somes at the end of the infection (at location 3, in Fig. 189) represents 

 another acquisition of the same type of immune response. The comple- 

 mentary action of the ablastic and trypanocidal effects not only effec- 

 tively suppresses the infection, but also prevents relapses and reinfections 

 for long periods (at least 325 days) . Thus a few trypanosomes left from 

 the initial infection or introduced by reinfection may be killed at once, 

 or, if they are not killed at once, their reproduction is inhibited until 

 they are killed. These statements hold for T. duttoni and, as far as they 

 have been tested, for T. nab'msi (previous citations). 



The three effects of immunity which operate at points 1, 2 and 3 in 

 Figure 189 are all due to humoral antibodies, as tested by passive transfer 

 experiments. They are associated with the globulin fraction of serum, 

 are acquired as a result of specific infection or specific immunization, and 

 are decreased in amount or delayed in time of appearance by splenectomy 

 and blockade. They differ in the following ways: The titer of the three 

 varies independently, as far as can be tested. The trypanocidal effects 

 are due to typical lysins which may, however, act as opsonins in vivo. 

 The trypanolysin which terminates the infection kills either adult or 

 dividing trypanosomes taken at any time during the course of infection, 

 whereas that which causes the first number crisis kills only those try- 

 panosomes which have just appeared in a rat's blood. The parasites that 

 survive the first number crisis are either basically nonsusceptible to this 



