550 IMMUNITY 



Active immunity is slow in its appearance, is more or less 

 dangerous to produce and is always attended with at least 

 some discomfort. It varies in the time it lasts but usually it 

 is quite persistent, lasting from a few weeks or months to 

 several years. Passive immunity is quite rapidly produced, is 

 attended with little or no danger and practically no discom- 

 fort. It is very limited in its period of duration. The most 

 extensive use of passive immunity is in immunizing children 

 against diphtheria. It is not employed to any great extent in 

 animal diseases. The immunizing of horses against tetanus 

 before an operation is, however, practiced by many operators. 



§ 446. Explanation of acquired immunity. A number 

 of ingenious explanations have been offered for acquired 

 immunity. The only ones that have withstood the test ot 

 their objectors are those of Metchnikoff, representing the cellu- 

 lar theory, and of Ehrlich, representing the humoral theory. 

 The results from different lines of investigation indicate that 

 the factors involved in securing immunity against infectious 

 diseases are multiple in number and varied in character. As 

 pointed out by Meltzer it is difficult to explain immunity as 

 being due to one or even to a few anti-bacterial properties of 

 the animal body. In the struggle against bacteria the defence 

 of the body is carried on by the united action of each and 

 every resisting influence. However, a number of theories 

 have been proposed. 



1 . The exhaustion theory. This theory was suggested in 

 1880 by Pasteur, who thought that the microorganisms grow- 

 ing in the body used up some substance essential to their 

 further existence and died out leaving the body unsuited for 

 future occupation. The theory could not apply to passive 

 immunity produced by the injection of antitoxin. 



2. The retention theory. This was proposed by Wernich 

 and Chauveau. This theory is based on the fact that bacteria 

 elaborate some metabolic product that inhibits their further 

 development and the future invasion in the tissues by the same 

 species. This theory is illustrated with the facts shown in 



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