CHAPTER XII 



ACQUIRED IMMUNITY 



Intra- and extra-cellular toxins Susceptibility to disease Methods of altering 

 virulence Parasites and saprophytes Smallpox Rabies Anthrax Diptheria 

 Active and passive immunity Antitoxin Syphilis Snake venom, and 

 vegetable poisons Yeast The nature of acquired immunity Pasteur's theory 

 Chauveau's theory The theory of neutralization The theory of habituation 

 Ehrlich's side-chain theory Inborn and acquired immunity. 



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397. f | ^HE nature of acquired immunity and the means by 

 which it is acquired have long been subjects of debate. 

 Many hypotheses have been formulated. Since the 

 mass of facts is enormous, and the difficulty of tracking their 

 relations very great, the subject is an exceedingly difficult and 

 complex one. Probably a workable idea of it will be most easily 

 conveyed to the reader if we begin by setting down an array of 

 facts, all of which I believe have been verified ; or, at any rate, few, 

 if any, of which are disputed. 



398. (a) Acquired immunity is evidently a reaction against toxins. 

 The toxins of the acute and sub-acute diseases, against which 

 alone immunity is acquired, are set free in the medium surrounding 

 the microbes much as snake venom is set free in the blood-stream 

 of the victim. Other toxins, intm-cellular toxins, are more or less 

 retained within the microbes. Thus, if the bacilli of tuberculosis, 

 the extra-cellular poisons of which are non-existent or very 

 feeble, be triturated in a mortar and treated with water, a poison, 

 tuberculin, may be obtained, which, when injected, produces 

 symptoms markedly different from those which occur in the actual 

 disease. () Some species of microbes can be cultivated in blood- 

 serum outside the body not only in the serum of susceptible 

 individuals, but also in the serum of those who have acquired 

 immunity. (V) Immunity is influenced by individual peculiarity, 

 race, and age. Thus many Englishmen are under ordinary con- 

 ditions immune to tuberculosis; "white rats, adult dogs, many 

 kinds of birds, and frogs are naturally immune to the bacillus of 

 anthrax, which is very fatal to cattle, common rats, field-mice, 



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