ECOLOGY, COMMENSALISM AND PARASITISM 363 



cells and so lead to ulcers and to abscess formation, as in amebic 

 dysentery. The disintegrating proteins of such cells produce toxins 

 which by autointoxication impair the vitality of the host. Less 

 frequently there are specific toxins which poison the host but in 

 only a few cases have such products been determined— sarcocystine 

 from the sporozoan Sarcocystis is the one toxin which has been 

 extracted (Laveran and Mesnil, 1899). 



Evidences of toxin action, both by direct poisoning and by 

 serological reactions of the host have been demonstrated in many 

 cases, rarely indicating direct secretion by the parasite (Sarcocystis 

 by Kasparek, 1895) but more often indicating a toxic compound 

 (endotoxin) which is liberated with death of the parasite or formed 

 as a chemical product from the breaking-down of substances com- 

 posing the body of the parasite (Endamoeba dysenteriae, Leishmania, 

 Trypanosoma, malaria organisms). In connection with host reac- 

 tions a great deal of work of serological nature has been done espe- 

 cially in reference to the detection of protozoan infections. Pre- 

 cipitation, agglutination, lysis and complement-fixation tests have 

 been developed to a high degree and are of the greatest importance 

 in detecting even mild infections (see Taliaferro, 1930, p. 411, for 

 serological methods). Craig (1926) has demonstrated two types of 

 toxin from Endamoeba dysenteriae: one, a hemolysin capable of 

 dissolving human red blood corpuscles; the other, a cytolysin cap- 

 able of breaking down the epithelial cells of the intestinal mucous 

 membrane of man and cats. Noguchi (1924) by serological methods 

 demonstrated the difference between morphologically identical 

 species of Leishmania (L. donovani, L. tropica and L. braziliense). 

 Parasiticidal reactions of the host to trypanosomes and malaria 

 organisms have been shown by Taliaferro (1926) and for trypano- 

 somes the parasite destroying agent was found, by experiments in 

 vitro, to be a lysin by Schilling (1902), Lingard (1904), Franke 



(1905) and Rodet and Vallet (1906), and by Massaglia (1911-1912) ; 

 also by experiments in vivo by Diesing (1905), Klein and Mollers 



(1906) and Johnson (1929). Similar reactions of the host cause a 

 diminution in rate of reproduction of the parasites, or even its 

 cessation (Taliaferro, 1924; Coventry, 1925). 



It is evident from the few references given above to a vast field 

 of protozoan research that definite and often specific changes occur 

 in the blood of individuals infected with different kinds of protozoan 

 parasites. If the results of such changes, in the form of parasiticidal 

 lysins, agglutinins, etc., are retained in the blood, they would be 

 effective against reinfection. Active immunity thus established by 

 infection in a host may be of longer or shorter duration, but for 

 the most part it lasts only for a short time. It appears to be a 

 potent protection in some types of Leishmaniasis, particularly that 

 produced by Leishmania tropica, where a localized ulceration confers 



