IMMUNITY AND ANAPHYLAXIS 181 



IMMUNITY. 



Immunity may be natural or acquired, and may be 

 considered under the following heads: 



Natural Immunity. Depends on (a) individual, (b) race, 

 and (c) species. 



Acquired Immunity. (a) By an attack of the specific 

 disease ; (b) By active immunization with living bacteria, 

 dead bacteria, or with toxins or filtrates ; (c) By passive 

 immunization with antitoxic serum, or antibacterial serum. 



Natural Immunity, or the resistance to bacteria con- 

 ferred by nature, is a characteristic of the living organism, 

 which varies with the individual, race, and species. Thus, 

 individuals vary greatly in their resistance to infection 

 from slight wounds, from polluted water and milk, and 

 from micro-organisms generally. The young animal is 

 usually less resistant than the mature of the same kind. 

 As regards race, negroes are noted for their high degree of 

 resistance to yellow fever, and in a less degree to malaria, 

 yet they quickly sicken and succumb to small-pox and 

 measles. Among animals, the Algerian sheep are more 

 highly resistant to anthrax than the European races. 

 The influence of species is seen in the non-liability of the 

 human to certain animal diseases, such as cattle plague, 

 fowl cholera, and swine erysipelas, whilst animals are 

 equally resistant to such human infections as cholera, 

 influenza, measles, etc. 



The causes of natural immunity are usually given as : 

 (i) The action of certain leucocytes and other cells in 

 engulfing and destroying the bacterial invaders, 

 called phagocytosis, and 

 (2) The action of the blood serum. 



I. Phagocytosis. Metchnikoff in 1884 advanced the 

 theory of phagocytosis, based on a careful study of the 

 subject, and since confirmed by many observers. The 

 phagocytes are in part wandering cells, and in part fixed 

 tissue cells. The chief wandering cells are the poly- 

 morphonuclear and large mononuclear leucocytes and 

 wandering tissue cells. Of the fixed phagocytes, possessing 

 the power of amoeboid movement, the cells lining the serous 

 and lymph spaces, the cells of the spleen pulp, and bone 



