THE PRINCIPLES OF 

 IMMUNOLOGY 



MUTUAL RELATIONS OF PARASITE AND HOST. 

 PARASITISM. 



VIRULENCE. 



METHOD OF DEMONSTRATION. 

 THE BASIS OF VIRULENCE. 

 ANIMAL PASSAGE. 



CAPSULE FORMATION. 

 AGGRESS1NS. 



POISONOUS SUBSTANCES OF BACTERIAL ORIGIN. 

 PTOMAINS. 



TRUE TOXINS (EXOTOXINS). 

 ENDOTOXINS. 



POISONOUS BACTERIAL PROTEINS. 

 ALTERATIONS OF VIRULENCE. 

 INCREASE OF VIRULENCE. 

 DECREASE OF VIRULENCE. 



Mutual Relations of Host and Parasite. The existence of in- 

 fectious disease depends fundamentally upon the invasion of a 

 plant or animal by some infective agent. The infective agent is 

 usually a microorganism either bacterial or protozoan in nature, 

 although infestations by larger organisms, such as worms within 

 the body or various forms of pediculi upon the surface, are some- 

 times spoken of as infections. In addition to bacteria the vegetable 

 world includes parasites, such as yeasts and fungi, which are cap- 

 able of producing disease. The actual production of disease depends 

 fundamentally upon the interrelationship between the infectious 

 agent and the invaded body. Bacteria are widely distributed in 

 nature, but the greater number of varieties have no capacity for the 

 production of disease. Those which produce disease are spoken of 

 as pathogenic, and those which do not produce disease are spoken 

 of as non-pathogenic. There are forms, however, which although 

 they ordinarily do not produce disease, may, under certain circum- 

 stances, develop this character. Animals and plants possess cer- 

 tain factors of resistance to the invasion of pathogenic organisms, 

 and the pathogenic organisms possess certain characters which 

 favor invasion. Both animals and plants live in constant associa- 

 tion with microorganisms, and apparently in many instances both 

 are benefited by this association. It is well known that certain 

 plants require for favorable development the association of the nitri- 

 fying bacteria. The intestinal canal of man, although free from 



