870 IMMUNOLOGY 



ways associated with the destruction of many organisms, and hence 

 involves a selection, but the selection is effective within a clone, i.e., 

 within the progeny of a single trypanosome. At the present time, how- 

 ever, it is impossible to decide whether such persistent modifications 

 are due to changes in gene constitution, or, if they are not, whether they 

 may eventually lead to such changes (see Robertson, 1929; W. H. Talia- 

 ferro and Huff, 1940; and, in part, Dobell, 1912). Nevertheless, they 

 are of extreme importance in allowing the parasite to overcome the 

 defensive processes of the host and are probably largely responsible for 

 the continued survival of the parasite. 



The lymphoid-macrophage system and particularly the macrophages 

 along the blood stream, appear to be involved in immunity, as indicated 

 by enlargement and histological changes in the spleen (Laveran, 1908; 

 Van den Branden, 1935; and others) and by the decreased length of 

 life of splenectomized animals infected with various trypanosomes (see 

 Davis, 1931, for most of the work prior to 1931; Nieschulz and Wawo- 

 Roentoe, 1930; Nieschulz and Bos, 1931; Russeff, 1935). Negative 

 results, as might be expected from the complexity of the problem as 

 explained previously, have also been reported by some of the earlier 

 workers (see Davis, 1931), and also by Davis (1931) and Browning 

 et al. (1934), whereas increased length of life was noted in partially 

 blockaded rats infected with T. equiperdum by Kolmer et al. (1933). 

 Data on splenectomized and blockaded animals which were treated are 

 omitted from consideration because the treatment itself may affect the 

 course of infection. 



Whether the trypanocidal antibody acts within the body as a trypano- 

 lysin, or as an opsonin with resulting phagocytosis, or both, has been 

 variously answered. Some authors have maintained that one or the other 

 is the sole method of defense; some that they share equal honors; and 

 some that, although lysis is the fundamental mechanism, phagocytosis 

 is responsible for clearing up the debris, and so forth. No one can 

 doubt the occurrence of phagocytosis after its careful description by 

 so many competent observers (Neporojny and Yakimoff, 1904; Sauer- 

 beck, 1905; Yakimoff, 1908; Mesnil and Brimont, 1909; Levaditi and 

 Mutermilch, 1910). On the other hand, W. H. Taliaferro and T. L. 

 Johnson (1926) reported the finding of disintegrating trypanosomes 



