106 Immunity 



EXPLANATION OF IMMUNITY. 



Before the facts now at our disposal had been gathered 

 together, and before the phenomena of immunity against 

 infection had been compared with those of intoxication, 

 Pasteur* and Klebsf endeavored to explain acquired im- 

 munity by supposing that micro-organisms living in the 

 infected animal used up some substance essential to their 

 existence, and so died out, leaving the soil unfit for further 

 occupation. This was known as the "exhaustion theory." 

 WernichJ and Chauveau thought it more probable that 

 the micro-organisms after having lived in the body left 

 behind them some substance inimical to their further 

 existence. This was known as the "retention theory." 

 These hypotheses are of historic interest only, and deserve 

 no more than passing mention, as they both fail to explain 

 natural immunity or immunity against intoxication. 



Karl Roser|| observed that the leukocytes of the bodies of 

 higher animals sometimes enclosed bacteria in their cyto- 

 plasm. Koch, Sternberg, and others confirmed the obser- 

 vation, but no attention was paid to it until Metschnikoff** 

 correlated it with other known facts and original observa- 

 tions, and came to the conclusion that the enclosed bacteria 

 had been eaten by the leukocytes in which they were killed 

 and digested, and that the behavior of the cells toward the 

 bacteria afforded an explanation of the mechanism by which 

 recovery from the infectious diseases takes place. The 

 original conception upon which this "theory of phagocyto- 

 sis" was founded, refers recovery in many, if not all of the 

 infectious diseases, to the successful destruction of the 

 invading bacteria by the body cells, especially the leukocytes. 

 These devouring cells Metschnikoff called phagocytes, and 

 of them he recognized two classes, the microphages, which 

 are white blood-corpuscles, and the macro p hages, which are 

 larger cells derived from the endothelial and other tissues. 

 Metschnikoff, his associates, and his pupils soon collected 



* "Compte rendu de la Soc. de Biol de Paris," xci. 



f "Arch. f. experimentele Path. u. Pharmak.," xm. 



J "Virchow's Archives," Bd. LXXVIII. 



"Compte rendu de la Soc. de Biol. de Paris," xc and xci. 



|| "Beitrage zur Biologic niederster Organismen." Inaugural Dis- 

 sertation, Marburg, 1881. 



** "Virchow's Archives," Bd. xcvi, p. 177; "Ann. de 1'Inst. Pas- 

 teur," t. i, p. 321, 1887. 



