THE DEFENSES OF THE BODY 269 



to a product of the infective organisms that is retained in 

 the tissues, and which, by its presence serves to prevent 

 the development of the same species of organisms should 

 they subsequently gain access to the tissues. This doctrine 

 is usually known as Chauveau's "Retention Hypothesis of 

 Immunity." We shall see later that it is only in small part, 

 if at all, a tenable theory. 



As opposed to Chauveau, Pasteur and his pupils, in the 

 same year (1880), expressed the opinion that acquired 

 immunity was to be explained in just the reverse way to 

 that conceived by Chauveau. They believed that in the 

 primary attack of infection something was extracted from 

 the tissues by the infecting organisms that was necessary to 

 support the growth -of the same species should it subse- 

 quently invade the body. This doctrine is known as Pas- 

 teur's "Exhaustion Hypothesis" of Immunity, and has 

 apparently little claim to serious consideration. 



Four years later (1884) Metchnikoff, while engaged upon 

 the study of certain lower forms of animal life, noticed 

 that particular mesodermal cells, in the course of their 

 wanderings through the body, had the power to actually 

 pick up, 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 generally known, are common not alone to the lower 

 forms of life, but to the most highly organized as well. In 

 the higher forms of animal life, the function of phogocytosis 

 is conspicuously ' exhibited by the wandering cells — i. e., 

 the white blood corpuscles. In a lower degree the inclusion 

 of foreign bodies with their subsequent digestion or disin- 

 tegration may occasionally be seen in other cells as well. 



Metchnikoff believed this phagocytic power to be the 



