IMMUNOLOGY 849 



L. G. Taliaferro (1929a, 1934c) intravenously superinfected birds and 

 monkeys with such large numbers of parasites that they could study the 

 superinfection quantitatively. They found that in immune animals the 

 parasites, after reinfection, begin to be removed at once, with the same 

 effectiveness that they are removed at the time of the crisis in initially 

 infected animals, and that in monkeys the asexual cycle occasionally ex- 

 hibits a transitory delay, accompanied by the production of fewer mero- 

 zoites. In other words, superinfection, since it occurs in an eflfectively 

 immune animal instead of in an uninfected animal, has no incubation 

 period or acute rise, but begins at once with a crisis and proceeds imme- 

 diately to latency. 



The problem now arises: what is the mechanism whereby the plas- 

 modia are removed throughout the course of the infection? The death 

 of all of the parasites, whether it takes place before, during, or after the 

 crisis (i.e., during natural or acquired immunity), is associated with 

 phagocytosis by macrophages, chiefly of the spleen, liver, and bone 

 marrow. Other cells play insignificant roles. Furthermore, the macro- 

 phages phagocytose free parasites, parasitized red blood cells, and resi- 

 dues of parasites such as malarial pigment and uninfected red cells, 

 which are probably injured by the infection. Malarial pigment is the most 

 indigestible part of the parasite red-cell complex, since it is often found 

 in macrophages months after all other vestiges of the parasites have 

 disappeared. Large monocytes containing malarial pigment may be found 

 in the peripheral blood, especially during the crisis and thereafter. A 

 review of the literature covering the considerable amount of work done 

 on this aspect of the subject will be found in W. H. Taliaferro and 

 Mulligan (1937) and involves direct evidence from necropsy findings 

 and indirect evidence from splenomegaly (see Stratman-Thomas, 1935; 

 Coggeshall, 1937; Afridi, 1938) and splenectomy and blockading 

 procedures. 



Although phagocytosis has been known for years to occur in malarial 

 infections. Cannon and W. H. Tahaferro (1931) and W. H. Taliaferro 

 and Cannon (1936) were the first to study infections and superinfec- 

 tions at closely spaced intervals and to correlate phagocytosis in detail 

 with the course of the infection and with immunity. The rate of phago- 

 cytosis during natural immunity, i.e., before the crisis, increases as the 

 number of parasites increases, but is always comparatively sluggish (PI. 



