126 VETERINARY BACTERIOLOGY 



hausted and the organism in consequence could grow no longer. 

 This theory was soon disproved. It was shown, for example, that 

 the Bacillus diphtheria would grow luxuriantly on sterilized blood 

 taken from a person immune to diphtheria. Chemical analyses 

 also showed that not all nutrients were exhausted from the medium 

 in the culture tube when the organism ceased growing. 



Noxious Retention Theory. Further study revealed the fact 

 that organisms ceased growing in a culture solution because they 

 produced substances deleterious to themselves. The organism 

 which sours milk, for example, produces acid until the concen- 

 tration is so great that it can no longer develop. The same is 

 true of yeasts in the production of alcohol, and 'of bacteria in the 

 transformation of alcohol to acetic acid. The nature of the 

 noxious material is known in very few cases. A logical explana- 

 tion of immunity seemed to be that something of the same 

 kind occurred in acquired immunity; that the organisms devel- 

 oped in the body until they produced so much material inimical 

 to their own growth that multiplication would cease, and the 

 individual would thereafter be immune. This theory was dis- 

 credited by subsequent workers, who proved that the substances 

 that prevented bacterial growth were produced by the body and 

 not by the bacteria. 



Metchnikoff's Theory of Phagocytosis. The theory was 

 advanced by Metchnikoff and his students that certain of the 

 body-cells, particularly the leukocytes, would take up, digest, and 

 destroy bacteria. Immunization accordingly consisted in a kind 

 of stimulation or training of the leukocytes to destroy the patho- 

 genic organisms. Cells capable of destroying microorganisms in 

 this manner he called phagocytes (Gr. phagein, to eat; kytos, cell). 

 Certain of Metchnikoff's ideas have not borne the test of time, but 

 in the main his theory still is used to account for immunity of 

 certain types. 



Ehrlich's Humoral Theory. Ehrlich has advanced the theory 

 that immunity is due to substances present in the body humors 

 which antagonize the growth and development of pathogenic 

 organisms or are capable of neutralizing their products. Such 

 substances capable of conferring immunity he calls antibodies. 

 and to their production by the body-cells he attributes the devel- 



