IMMUNOLOGY 871 



in the blood during experimental crises, which they interpreted as 

 stages in lysis. This question can probably be answered as was the simi- 

 lar question with regard to the trypanolysins in T. lewisi infections. 

 The trypanosomes become sensitized by antibody, and the process may 

 be completed by lysis (extracellular enzymes) on one hand, or by diges- 

 tion within phagocytes (intracellular enzymes) on the other hand. Which 

 occurs may depend to a certain extent on the strength of the antibody. 



Besides the cellular basis for the production of the trypanocidal anti- 

 body and the cooperation of phagocytes in removing sensitized para- 

 sites, Kuhn (1938) has shown a peculiar role of the lymphoid-macro- 

 phage system in passive transfer of anti T. equiperdum protective serum 

 to mice. Thus immune serum, which is effective in protecting normal 

 mice in doses of 0.4 cc. per 20 gm. body weight, gives only partial pro- 

 tection to splenectomized mice, blockaded with India ink, in doses as 

 high as 1.7 cc. per 20 gm. body weight. Suitable experiments indicate 

 (as in previous work with T. lewisi) that this finding is due neither to 

 the lowering of complement nor to the removal of phagocytic cells 

 which might be necessary in removing opsonized parasites, but rather 

 to the prevention of antibody uniting with trypanosomes. An interest- 

 ing, but confusing element in these experiments was that unilateral 

 nephrectomy or ureterotomy was accompanied by a slight reduction in 

 the protective titer of the serum. 



A comparison of the resistance acquired by hosts against pathogenic 

 and against nonpathogenic trypanosomes is very illuminating. In the 

 first case, the host acquires practically no resistance (mouse) or it pe- 

 riodically forms trypanolysins (guinea pig, dog, etc.) which hardly 

 ever effectively rid the animal of infection because a few of the patho- 

 gens generally become resistant and repopulate the blood again and 

 again until the host dies. In the second case, the host first produces an 

 antibody which inhibits cell division of the parasites and then periodi- 

 cally forms trypanolysins which get rid of the nonreproducing parasites. 



Practical Applications of Immune Reactions 



By far the most extensive literature on the immunology of the para- 

 sitic Protozoa deals with experiments fundamentally planned in the hope 

 of achieving some practical method of preventing, curing, or diagnosing 



