THEORIES OF IMMUNITY 117 



When an invading microorganism has reached a suitable atrium of 

 the body, overcome the initial defense of the host at that point, and 

 has successfully resisted the normal humoral or cellular opposition 

 of the host, a new phase of the struggle becomes prominent, during 

 which the host gradually develops a specific attack upon the invader, 

 bringing into action latent forces which constitute the third and last 

 defense of the body. The invader also may change its weapons to 

 some degree to meet the antimicrobic activity of the host and the 

 result of the struggle may be complete recovery from infection, chronic 

 disease, the bacillus carrier state, or death of the host. 



The initial and secondary defensive powers of the host, therefore, 

 are both cellular and humoral in character. The intact skin and 

 mucous membranes of the gastro-intestinal, respiratory and genito- 

 urinary tracts are important initial non-specific lines of defense. The 

 phagocytic activity of leukocytes and certain fixed tissue cells, and 

 the natural, normal bactericidal substances of the blood and lymph, 

 which bathe the initial line of defense, are important adjuvants in 

 maintaining the integrity of these initial barriers to infection. In 

 certain infections the humoral factors are the more important, while 

 in others the cellular mechanism is conspicuous. 



The defensive mechanism against the same bacterium may be 

 different in one or another animal. For example, dogs and rats are 

 relatively immune to infection with the anthrax bacillus. The immu- 

 nity observed in the dog appears to be due to phagocytic activity of 

 leukocytes which engulf and destroy anthrax bacilli which may have 

 gained entrance to the body. 1 The rat, on the contrary, enjoys immu- 

 nity not because its leukocytes engulf and destroy anthrax bacilli; 

 the blood of the rat possesses soluble, non-specific bactericidal sub- 

 stances which destroy anthrax bacilli. Frequently both the cellular 

 and humoral elements are engaged either simultaneously or succes- 

 sively as the struggle between host and invading organism proceeds. 



m. THEORIES OF IMMUNITY. 



Two distinct explanations have been advanced to account for the 

 mechanism of immunity as it is observed during the course of disease : 

 the cellular or phagocytic theory championed by Metchnikoff and 

 his followers, and the humoral theory developed by Ehrlich. 



Both of these theories, the cellular and the humoral, have in com- 



1 Hektoen, Jour. Am. Med. Assn., 1906, xlvi, 1407. 



