INFECTION AND IMMUNITY. 559 



extracted from the tissues by the infecting organisms that 

 was necessary to support the growth of the same species 

 should it subsequently invade the body. This doc- 

 trine is known as Pasteur's " Exhaustion Hypothesis " 

 of Immunity, and has apparently little claim to serious 

 consideration. 



Four years later (1884) Metschnikoff, while engaged 

 upon the study of certain lower forms of animal life, 

 noticed that particular mesodermal cells had the power 

 to actually pick up, in the course of their wanderings 

 through the body, insoluble particles that had gained 

 access to it in one way or another. He looked 

 upon them as functioning, therefore, as scavengers. 

 These phagocytes, as they are now known, are common 

 not alone to the lower forms of life, but to the most 

 highly organized as well ; and in so far as they concern 

 man, they are generally known as leucocytes, or white 

 blood corpuscles. In a lower degree the function of 

 including foreign bodies with their subsequent degenera- 

 tion or disintegration may occasionally be seen in other 

 cells as well. 



Metschnikoff believed this phagocytic power i. e. y 

 the property of devouring, possessed by the white blood 

 corpuscles, to be the most important defensive mechan- 

 ism possessed by the body, and believed both natural 

 and acquired immunity to be referable to it ; in the for- 

 mer case regarding it as a natural endowment, in the 

 latter as a function that had been excited by the 

 specific stimulus offered by the organisms or their 

 poisons that were concerned in the primary attack of 

 disease from which the animal recovered, or by the 

 organisms used in purposely exciting a modified form 



