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CHAPTER III. 



Immunity and Heredity. 



In this chapter I shcall try to give a sketch of the respective theories of 

 immunity and heredity, both of which have a large influence on disease. 

 Susceptibility is, of course, the opposite of immunity. 



Immunity. 



Immunity is the power which an individual or a species has to resist the 

 attack of a disease. It may be racial, individual, or acquired ; and may be 

 absolute or comparative. For instance, horses possess racial immunity against 

 the scarlet fever of man and the pleuro-pneumonia of cattle. Men are 

 immune from strangles ; and ruminants from glanders. Among every 

 species of animal, we find individuals which are more or less refractory to the 

 inroads of a disease to which their fellows show marked susceptibility. The 

 man, hor&e, or ox which has had one attack, respectively, of smallpox, 

 strangles, or pleuro-pneumonia, will have acquired more or less immunity 

 from a second attack. This acquired immunity is never absolute. 



As possessors of comparative immunity, I may cite Algerian sheep, which 

 are refractory to anthrax, when inoculated in the usual way ; but prove 

 susceptible to it when the dose of the virus is largely increased. 



Dogs rarely contract tetanus; horfes, on the other hand, are very sus- 

 ceptible to it. 



On page 125, I allude to the accidental immunity of the carnivora from 

 actinomycosis. 



The immunity of fowl against anthrax appears to be due to their high 

 internal temperature (107.5° F.) ; for Pasteur has proved that if a fowl be 

 inoculated with anthrax, and is then placed and kept in water at a tempera- 

 ture of 77° F., so as to considerably reduce its internal heat, it will die of 

 anthrax in about a day and a half. 



The nature of the lesions set up in the tissues by certain diseases, greatly 

 ati'ects comparative resistance. Thus, in a man who has been inoculated 

 with anthrax, the virus is more or less arrested at the seat of inoculation in 

 the malignant pustule which forms at that spot; but in the horse it spreads, 

 apparently unchecked, throughout his entire body. Hence, anthrax is far 

 more fatal, and runs a much more rapid course in horses than in men. 



In those diseases in which one attack has a well-marked power of con- 

 ferring subsequent immunity, protection, according to Metchnikoff, is acquired 

 by the previous training in devouring microbes which the leucocytes have 

 undergone. " If we inoculate the virus of anthrax, for example, under the 

 skin of an ordinary rabbit and of a rabbit which has been protected by 

 vaccination, behold what we may observe at the respective seats of inocula- 

 tion. In the ordinary rabbit, the microbes multiply rapidly. The swollen 

 part is full of watery fluid, and is poor in cells. Little by little the swelling 



