SECTION VI 

 RESISTANCE AND IMMUNITY 



CHAPTER XXII 

 THE BLOOD AND LYMPH AS PROTECTIVE MECHANISMS' 



General Consideration. — The phenomena of resistance and 

 immunity may justly be regarded as belonging to physiology, because 

 the protection afforded an animal against toxic influences of all kinds, 

 finds its origin in certain agents which are generated by the components 

 of its tissues. In a practical way, however, the subject of immunity 

 is more intimately related to bacteriology, and hence, it is intended to 

 restrict the present discussion to the most essential general facts. 



The body of an animal is protected in a certain measure against 

 various poisonous substances, albuminous material, ferments, cellular 

 products, and pathogenic bacteria and their derivatives. When fully 

 developed, a resistance of this kind constitutes an immunity. Thus, 

 an animal which cannot be affected by an ordinary dose of a certain 

 toxic substance, is said to be immune against it. This resistance, 

 however, may not be sufficient against larger doses of the same poison, 

 nor against minimal doses of an unusually virulent toxin. Moreover, 

 the degree of immunity may be varied by outside influences so that 

 periods of resistance alternate with periods of susceptibility. 



Immunity may be partial or complete, but the absolute type is rather 

 uncommon. Thus, cold-blooded animals are often quite insusceptible 

 to many of the bacteria which produce violent reactions in warm- 

 blooded animals, while many of the latter are thoroughly protected 

 against the infections which the lower forms are very prone to incur. 

 As far as man is concerned, an immunity may be restricted to (a) all 

 individuals, (6) certain races or tribes, (c) certain families, and {d) cer- 

 tain persons. For example, such diseases as Texas fever or hog cholera 

 are not prevalent among mankind, while yellow fever and malaria do 

 not usually attack the negroes of the West Indies. Quite similarly, 

 certain families or individuals may possess a degree of resistance 

 against tuberculosis which is not shown by others. 



Like other biological characteristics, the power of resisting a 

 specific pathogenic influence may be propagated from parent to off- 



^A more detailed account will be found in Wassermann's ''Immune sera, 

 hemolysins, cytotoxins and precipitins," translated by Bolduan, New York, 1904. 



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