CHAPTER XIII. 

 IMMUNITY REACTIONS. 



THAT the organism is overwhelmed by a large dose of poison but 

 recovers from a small one should not particularly surprise us. Ever 

 since the recognition of the nature of infectious diseases, it must have 

 amazed biologists that every infected organism did not succumb to 

 the slightest infection. Microorganisms multiply indefinitely, and, 

 theoretically, it is only a question of hours before the number present 

 shall be overwhelming whether the infection is with a large or a small 

 dose. Were this assumption, to which we might be led from the ob- 

 servation of culture media, correct, no living thing, plant or animal, 

 could exist. There must be inherent forces in the living organism 

 which protect it against pathogenic germs, which make it immune to 

 such injuries, and which are called, accordingly, immune bodies 

 (immune substances). 



L. PASTEUR was the pioneer in the systematic study of immunity. 

 He produced experimental proof that immunity might be artificially 

 produced by previous treatment with attenuated infective agents 

 (chicken cholera) just as had been done previously, in the case of 

 vaccination against smallpox. These investigations received a 

 mighty impulse when ROBERT KOCH succeeded in growing disease 

 germs in pure culture. The doctrines of immunity and predisposition 

 were developed into a special branch of science which at present 

 holds the chief interest of scientific medicine. 



It was recognized that the body could overcome its invaders in 

 various ways: substances occur which make bacteria harmless by 

 dissolving them, bacteriolysins (acting against vibrios, e.g., of cholera, 

 and against typhoid), and others which clump them together and 

 precipitate them, the agglutinins (against typhoid, paratyphoid, 

 dysentery, etc.). In other cases, the protection is directed prin- 

 cipally against the poisons, toxins, which the organized germs de- 

 velop (diphtheria toxin, tetanotoxin, etc.). The organism possesses 

 a peculiar protective mechanism in the leucocytes, which take up 

 and digest the bacteria and cocci, devouring them like free living 

 amebae in search of food. This phenomenon, which was recognized 

 and studied chiefly by E. METSCHNIKOFF, is called phagocytosis. 



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