Explanation of Immunity iii 



it is slowly broken down by enzymic action, but that the cells, 

 having once acquired the property of destroying it, seize eagerly 

 upon the protein the next time it is offered, disintegrate it rapidly, 

 and so disseminate throughout the body the degradation products, 

 some of which may be toxic and account for the reaction. 



Anaphylaxis is not a disturbance of the cells of the body, as 

 some have thought, but is at least in part a disturbance of the compo- 

 sition of the blood, as can be shown by the occurrence of what is 

 known as passive anaphylaxis. If the blood-serum of a senitized 

 animal be withdrawn and injected into a normal animal of the same 

 kind, it carries the sensitization with it. The new animal, however, 

 does not become sensitized at once, but only after some days, hence 

 it is equally true that the disturbance is not solely in the blood, else 

 why should not the sensitization be immediately present upon the 

 injection of the serum? 



Anaphylaxis may, furthermore, be local. Thus, when certain 

 substances hke tuberculin are dropped in the eye there is no effect, 

 but when a second application is made, after some weeks, the eye 

 may be reddened. 



Anaphylaxis may play a r61e in infection. In cases where an 

 attack of an infectious disease leaves no immunity, the body may 

 be left h)^ersensitive to subsequent attacks. 



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 immunity 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." Wer- 

 nich| 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 Roserll observed that the leukocytes of the bodies of higher 

 animals sometimes enclosed bacteria in their cytoplasm. Koch, 

 Sternberg, and others, confirmed the observation, but no attention 

 was paid to it until Metchnikoff** correlated it with other known 



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



t "Arch. f. experimentelle Path. u. Pharmak.," Xlii. 



t "Virchow's Archives," Bd. lxxviii. 



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



II "Beitrage zur Biologie niederster Organismen," Inaugural Dissertation, 

 Marburg, 1881. 



** "Virchow's Archives," Bd. xcvi, p. 177; "Ann. de I'Inst. Pasteur," 1887, 

 t. I, p. 321. 



