114 IMMUNITY. 



Immunity may be : (a) natural, the immunity of certain 

 individuals and races against certain diseases at all times ; 

 (6) inherited, the transmission of immunity from the mother 

 to the foetus through the placenta, or from the mother to the 

 child through the milk ; (c) acquired, immunity produced by 

 one attack of the disease, or by inoculation, or by the intro- 

 duction into the body of an artificially prepared antitoxin. 



Immunity may also be: (1) passive or (2) active. 



Immunity is said to be active when the tissues of the body 

 possess the resisting power. 



In passive immunity the resisting power resides only in the 

 blood. It may be produced by injecting the blood-serum of 

 an animal which is actively immune. 



The immunizing substance always is stored up in greatest 

 quantity and for the greatest length of time in the organ 

 or tissue which would be most seriously affected by the 

 disease. 



NATURAL IMMUNITY : It is impossible to give an exact 

 definition of immunity, especially natural immunity. Natural 

 immunity is not absolute, although it is more so than the 

 acquired form. The cause of immunity is still a matter of 

 conjecture. The most we can do is to study the conditions 

 which influence immunity either one way or the other. 

 Various theories as to its nature have been suggested : 



Phagocytosis : The phagocytic theory of immunity was 

 advanced by Metschnikoff. it is based on a peculiar property 

 possessed by the white corpuscles, especially the polymorpho- 

 nuclear leucocytes, and certain fixed tissue-cells, the endothe- 

 lial cells. The former are termed microphages, and the latter 

 macrophages. These cells take up or devour inert particles, 

 hence their name of phagocytes. In an inflammation, the 

 leucocytes rush to the seat of the inflammation, and not only 

 wall off the process, but also assist in disposing of the effete 

 or foreign matter. Leucocytosis (an increased number of 

 leucocytes) is a prominent feature in nearly all the infectious 

 diseases in fact, the prognosis is influenced largely by the 

 degree of leucocytosis. Metschnikoff observed that when an 

 animal is injected with a culture of bacteria, the leucocytes 

 are subsequently found to contaiiT these organisms ; further, 



