144 IMMUNITY. THEORIES OF IMMUNITY 



nikoff, with his theory of phagocytosis, whereas Ehrlich is the father and 

 leader of the latter, with that marvelous invention of human ingenuity, 

 the side-chain theory. 



THEORIES OF IMMUNITY 



The earlier hypotheses advanced by various investigators are now 

 only of historic interest, as in the light of subsequent discoveries and 

 observations they have failed to offer adequate explanations. 



Pasteur's own theory and explanation of the mechanism of acquired 

 immunity sought to show that the microorganisms living in the infected 

 animal used up some substance essential to their existence, so that, for 

 lack of proper nourishment, the microorganisms were eventually forced 

 to retire, the soil being unfit for further occupation. This was known 

 as the "exhaustion theory" 



Chauveau considered it more probable that the microorganisms, 

 after having lived in the body of an infected animal, produced sub- 

 stances that, accumulating in the blood, had an inhibitory action on the 

 bacteria and were inimical to their further existence. This was known 

 as the "retention theory," and in some particulars was just the opposite 

 of the exhaustion theory. 



THE THEORY OF PHAGOCYTOSIS 



In 1883, when Metchnikoff showed that certain of the body-cells, 

 and, particularly, the polynuclear leukocytes, were active in the defense 

 of the human body against invasion by microparasites, real light was 

 thrown upon the unknown problems of immunity. Although he has 

 since amplified his theory, as new theories were adduced to describe the 

 part played by the body-fluids and the organisms themselves, yet his 

 theory of phagocytosis remains a demonstrable fact, and establishes the 

 important role of cells in the processes of immunity. 



According to this theory, certain of the body-cells are able to ingest 

 an infecting parasite, a red corpuscle, or other cell in the same manner 

 as an ameba ingests a food-particle, and to dispose of it by intracellular 

 digestion through the agency of ferments known as "cytases." To 

 such cells Metchnikoff applied the name phagocyte, as he likened them 

 to scavengers, i. e., they were concerned in picking up and disposing of 

 offensive material, both living and dead. 



Various body-cells are capable of becoming phagocytes. The poly- 

 nuclear leukocytes are particularly active in acute infections, and have 

 been called microphages. Endothelial cells, mononuclear leukocytes, 



