Immunity and Resistance 641 



ablastin effect) appears after about four days in a primary infection and 

 the titre increases almost explosively toward the end of the first week 

 (50). The ability to produce ablastin is well marked in animals of 25 

 days or older. Young rats apparently produce little or no ablastin and 

 often die with T. lewisi infections (60). Production of ablastin also may 

 depend upon the diet of the host. A pantothenate deficiency almost 

 doubles the period of multiplication in T. lewisi infections (17, 20), and 

 a biotin deficiency likewise delays the production of ablastin as well as 

 trypanolysin (40). Irradiation of rats with X-rays (120) and dosage with 

 sodium salicylate (15) also delay the ablastin effect. 



The first crisis is caused by a lysin, which upon passive transfer to rats 

 with early infections, induces a crisis within a few hours (51). The second 

 crisis, according to different suggestions, depends primarily upon the 

 action of a new lysin (167), mainly upon phagocytosis (137), or perhaps 

 upon both factors. 



The relative importance of cellular mechanisms in resistance to trypa- 

 nosomiasis is uncertain, but the possible significance of phagocytes in 

 general has been considered by various workers. Phagocytosis has been 

 reported in vitro and in vivo. Ingested flagellates are sometimes seen in 

 circulating leucocytes, chiefly the large mononuclears, and also in fixed 

 tissue phagocytes. Furthermore, rabbits which survive infections with 

 T. brucei show an increase in percentage and in total number of mono- 

 cytes just before the first crisis. Rats and non-resistant rabbits show no 

 such increase (81). An absolute monocytosis also occurs about the time 

 of the first crisis in rats which survive T. lewisi infections, but not in 

 those which are to die (64). In addition to any possible importance in 

 phagocytosis, the lymphoid-macrophage cells have been considered as 

 a source of ablastin — for example, by Regendanz and Kikuth (137) who 

 noted that splenectomy usually delayed the ablastin effect for several 

 days and was sometimes followed by death. Others (175) have failed to 

 detect any marked effect of splenectomy on the production of ablastin. 

 A possible relation of lymphoid-macrophage cells to formation of lysins 

 also has been considered. Denison (63), for instance, has traced the effects 

 of blockade with trypan blue upon the production of T. cruzi lysin in 

 rats. In hanging-drop preparations, antiserum from infected normals 

 produced lysis much more rapidly than that from blockaded animals, and 

 the antibody titre was higher in the former serum. 



Defensive mechanisms in malaria 



Early investigations on bird malaria (reviews: 168, 171) showed 

 that the incubation period — in canaries infected with Plasmodium 

 cathemerium, for instance — is followed by an acute stage in which 30-50 

 per cent of the corpuscles are invaded. If the bird survives, the acute 

 phase is terminated by a crisis which eliminates most of the parasites 



