INFEOTION AND IMMUNITY. 543 



It is well known that among man and the lower ani- 

 mals individuals are frequently encountered who are in 

 general less susceptible to infection than are others of 

 their species ; and that particular species of animals not 

 only do not suffer naturally from certain specific dis- 

 eases, but resist all efforts to produce the diseases in 

 them by artificial methods; in other words, they are 

 naturally immune from them. The term " natural im- 

 munity," as now employed, inij^lies a congenital condi- 

 tion of the individual or species, a condition peculiar to 

 his idioplasm, which has been transmitted to him as a 

 tissue-characteristic through generations of progenitors. 



Again, it is often observed that an individual or an 

 animal after having recovered from certain forms of 

 infection has thereby acquired protection from subse- 

 quent attacks of like character; in other words, they 

 are said to have acquired immunity from this disease. 

 "Acquired immunity" implies, therefore, a condition 

 of the tissues of an individual, not of necessity peculiar 

 to other members of the race or species, that has 

 originated during his life from the stimulation of his 

 integral cells by one or another of the specific, infective 

 irritants that may have been purposely introduced, or 

 accidentally gained access to his body. 



The problem involving the explanation of the fore- 

 going interesting ideas and observations has afforded 

 material for reflection and hypothesis for a long time. 

 It is only through investigations conducted during re- 

 cent years that it has met with anything approaching 

 satisfactory solution, and even now there remain a 

 number of important points that are veiled in obscurity. 



Conspicuous among the observers who have en- 

 deavored to explain the mechanism of immunity may 



