IMMUNITY IN EXPERIMENTAL INFECTIONS 135 



the iiu'i-eased ability of the mononuclear phajiocytes to ensiili" and 

 destroy typhoid bacteria. Cannon and Taliaferro (1931) carried 

 out an extensive investigation of the cellular reactions in the tis- 

 sues of canaries infected for the first time with Plasmodium cathcr- 

 ncnum and also in superinfected birds. This last observation in- 

 dicates that the i^rimary infection alters the reactivity of the 

 luescnchyiuo so that ])lui.<i'<)('yt()sis is more i'a])id wlicn a secondary 

 infection is produced. 



In 1936 Taliaferro and Cannon re]>orted u])on the cellular 

 reactions during primary infections and superinfections of Plas- 

 modium Jirdsilianum in Panamanian monkeys. They say that 

 the maci'ophages of the s]deen. liver and bone marrow are the 

 ])rimary defense against malaria. They suggest that tlie reason 

 for this is probably the slow circulation of the blood through these 

 tissues which allows for direct contact of the blood and macro- 

 phages wherea.s elsewhere in tlie body similar conditions do not 

 exist. 



Tissue Resistance to Cysticekcus pisiformis. — In a study of 

 experimental infections of C iisticerciis pisiformis, a larval ta]ie- 

 worm, in passively inuuunized rabbits, Leonard (1940) showed 

 that host resistance is ex])ressed parenterally as aii enhanced 

 and accelerated tissue response; larvae in the liver were killed 

 in these animals in aj^proximately one-half the time required for 

 their death in normal animals. Leonard and Leonard , (1941) 

 found that in passively imnuuiized rabbits, the intestinal wall, 

 or some substance released by the host tissues within the intestine, 

 plays a major part in the total resistance mechanism. This was 

 shown by injecting artificially hatclied larvae into tlic nu\senteric 

 veins, thus allowing them to I'cacli the liver witliout ]ienetrating 

 the intestine. It was shown by this means that, in ])assively im- 

 nuuiized animals, only 3 to 4 pei* cent of the larvae are capable of 

 overcoming the resistance of the host intestine, while in normal 

 animals, 85 per cent of the larvae are able to penetrate the in- 

 testine and safely reach the liver. 



Removal of Bacteria from the Blood Stream. — Cannon, Sulli- 

 van and Xeckermann* (1932) investigated the conditions in- 



*Cannon, P. R., SuUivan, F. L., ami Xeckennani), E. F. : J. Expor. Meil. 5.'»: 

 121, 1932. 



