142 IMMUNITY. THEORIES OF IMMUNITY 



Even superficial observation shows that not all persons are equally 

 susceptible to a given disease, and during the course of epidemics it will 

 be seen that some individuals, although freely exposed, escape infection. 

 Certain species of animals may likewise display a uniform resistance to 

 an infection that will readily enough attack another species of the same 

 general family. It has been demonstrated experimentally that a certain 

 pathogenic bacterium will produce a severe infection in one species of 

 animals and not in another. It may frequently be noticed that even 

 though an infection occurs, it is readily overcome by the natural resources 

 of the host, the latter escaping with slight or no symptoms of disease. 

 In other words, certain persons and animals apparently possess a natural 

 resistance or immunity to disease, which may be general, non-specific, 

 or due to specific antibodies, this type of immunity being frequently 

 relative and seldom absolute. 



Definition. Immunity, therefore, in a broad sense, is the effective 

 resistance of the organism against any deleterious influence; in the usual 

 and more restricted meaning the term is applied to resistance against in- 

 fection with vegetable and animal parasites and their products, which are 

 pathogenic for other animals of the same or of different species. 



It should be remembered that immunity means not only the ability 

 to resist an infection or successful invasion of the tissues by micropara- 

 sites, but also the continual resistance offered as long as the infection 

 lasts; that is, immunity implies not only resistance to the onset of in- 

 fection, but also to the course and progress of the resulting infectious 

 disease. The science of immunity has, therefore, for its object the study 

 of the mechanism of resistance to and recovery from an infection. 



HISTORIC 



The development of the science of immunity forms one of the most 

 interesting chapters in the history of medicine. Even in ancient history 

 we can trace the conception of our modern ideas on immunization. 



Hippocrates taught that the factor that causes a disease is also 

 capable of curing it practically the same theory as the more modern 

 homeopathic doctrine of (( similia similibus curantur." Pliny the Elder 

 recommended the livers of mad dogs as a cure for hydrophobia, thus 

 coming very near to the basis of the Pasteur discovery. As was pointed 

 out by Elizabeth Fraser, the same idea is expounded in the mythologic 

 tale of Telephus, who cured his wound by applying rust from the sword 

 which inflicted it, and in the story of Mithridates, King of Pont us (B. 



